The Yanghe Reservoir has been the important drinking resource of Qinhuangdao and Beidaihe summer central office. However, in recent decades the water body has been undergoing intense eutrophication, resulting in loss of water quality and increases in the frequency of cyanobacterial bloom outbreak. It almost lost the function as drinking resource during the occurrence of algal bloom. We have analyzed the eutrophication development and its key regulating factors of the reservoir by collecting history materials and monitoring dataset of water quality in 2011. The results showed that the concentrations of total nitrogen (TN), total phosphorus (TP) have been higher continuously than the critical ones of eutrophication during 1990-2011. Algal blooms have almost broken out in August of each year. Based on the monitoring data in August of each year, TN, TP, chlorophyll a (Chl-a) increased significantly since 1990, their concentration increased from 0.72 mg/L, 0.029 mg/L, 3.35 μg/L in 1990 to 4.25 mg/L, 0.103 mg/L, 88.5 μg/L in 2011, respectively. Secchi disk depth (SD) decreased significantly, and it was 1.65 m in 1990, while it decreased into 0.3 m in 2011. Trophic state index based on TN (TSI_TN), TP (TSI_TP), SD (TSI_SD) and Chl-a (TSI_CHL) were also increased rapidly, and from 49.71, 52.71, 52.78, 42.46 in 1990 to 75.33, 70.98, 77.36 and 74.58 in 2011, respectively. Based on the Carlson-type TSI, the trophic state of the system were mesoeutrophic in 1990 and became hypereutrophic in 2011. There was a good relationship between SD and Chl-a during 1990-2011 (R²=0.9186), indicating that algae dominate light attenuation. From May to October in 2011, water temperature (T) varied from 15.7 to 29.2 ℃, with mean (22.75±4.77) ℃; SD was 0.30-4.00 m with mean (1.66±0.98) m; dissolved oxygen (DO) was 5.38-13.58 mg/L with mean (8.91±2.09) mg/L; pH was 8.17-10.92 with mean (8.87±0.78); chemical oxygen demand (COD_Mn) was 2.06-4.90 mg/L with mean (3.52±0.94) mg/L; total organic carbon (TOC) was 8.26-23.16 mg/L with mean (13.57±3.66) mg/L; TP was 0.0336-0.095 mg/L with mean (0.060±0.014) mg/L; TN was 3.078-6.752 mg/L with (4.713±1.014) mg/L; nitrate nitrogen (NO₃-N) was 0.393-2.856 mg/L with mean (1.252±0.788) mg/L; ammoniacal nitrogen (NH₄-N) was 0.0077-0.2191 mg/L with mean (0.109±0.070) mg/L; orthophosphate (PO₃-P) was 0.0022-0.0096 mg/L with mean (0.0066±0.0022) mg/L; Chl-a was 1.46-136.97 μg/L with mean (27.31±34.99) μg/L. It revealed that the concentration of each factor varied substantially among months. The correlation relationship analysis showed that T, SD, DO, pH, TP, PO₃-P, COD_Mn and TOC have obvious correlation with Chl-a. SD, DO and pH were the results induced by water eutrophication, but not the driving factors, therefore, SD, DO and pH were the major symptom of water quality deterioration induced by algal bloom outbreak. Since the reservoir is situated in northern temperature zone with 2-3 months ice cover, T should be a driving factor of summer blooms. The regression equation of log-transformed values for Chl-a on the other factors was performed and it is presented as below: log(Chl-a) =-1.768+0.058T+0.006SD-0.067DO+0.068pH+0.118TN-0.091NO₃-N+1.25NH₄-N+2.272TP+11.669PO₃-P+0.037COD_Mn+0.046TOC, R²=0.979. During the outbreak of algal bloom from the middle July to the end of August in 2011, T was 23.5-29.2℃, and the dominant species were Microystis aeruginosa and M. ichthyoblabe that belong to Cyanophyta with cell density of 3.5×10⁶ and 1.4×10⁶ cells/ml, respectively. The Yanghe Reservoir was a typical lake-reservoir with slow water flow and long residence time. In order to prevent the outbreak of algal bloom and improve the function as drinking resource, the nutrient concentrations must be decreased by reducing exogenous and control endogenesis pollution, and simultaneously change hydrodanimic conditions by increasing flow velocity or water exchange speed.