Wheat is the second most planted crop in the world and plays an important role in China and the world's food security. Aphids are a serious insect pest group for the worldwide wheat growing area and pose a critical threat to wheat yield. It is known that in modern agriculture crop yield largely depends upon fertilizer input, whereas the occurrence of insect herbivores is closely related to the nutrient status in the plant tissues supplied by soil fertility. The prevailing view is that a high potassium level in plant tissues supplied by the soil decreases the occurrence of many pests. Our previous research showed that potassium-deficient wheat had the largest aphid population, whereas the presence of sufficient potassium significantly reduced the aphid population in wheat. So, understanding the relationship between fertilization and the incidence of insect pests is essential for the management of chemical fertilization and insect pests in the modern agroecosystem. In this paper, the effects of potassium supply on signal molecular contents and defense enzymes activities of wheat leaves infested by aphids (Sitobion avenae) were investigated. Solution culture was used in the experiment and two potassium (K) levels were applied, 2 mmol/L and 0.005 mmol/L KCl, respectively. After 20 d, wheat seedlings were infested by aphids and the signal molecular contents and defense enzymes activities were analyzed over different infestation times. The results showed that K deficiency significantly reduced jasmonic acid (JA) and salicylic acid (SA) content in wheat leaves and increased lipoxygenase (LOX) and polyphenoloxidase (PPO) activities, whereas it had no effects on phenylalnine ammonialyase (PAL) and peroxidase (POD) activities. After 48 h aphid infested, JA content in wheat grown in 2 mM KCl was significantly increased, 32.2% higher than the no aphid infested control and 50.0% higher than the K-deficient wheat. SA content in K-deficient wheat was significantly increased after aphid infestation, whereas there was no significant change in K-sufficient wheat. High levels of potassium significantly increased the activity of LOX after 24 h aphid infestation and reached the highest activity at 48 h which was 3.6-fold higher than the control. After 72 h aphid infestation, PPO activity in K-sufficient wheat was increased by 25.8% greater than the control. PAL activity in K-sufficient wheat was significantly increased after 24 h aphid infestation and was 69.9% higher than the control. Aphid infestation significantly increased POD activity of K-sufficient wheat and the activity was 3.0-fold higher than the control at 72 h. However, there were no striking changes in those four enzymes activities in K-deficient wheat infested by aphids. It is indicated that appropriate K applied can significantly increase the content of jasmonic acid, activate JA-signalling transduction pathway, as well as improve defense enzymes activities in wheat after aphid infestation, thereby enhancing the resistance of wheat to aphids. Since PAL, the first and key enzyme in the phenylpropanoid metabolic pathway, catalyzes phenylalanine into cinnamic acid which is the substrate of SA synthesis. We could not determine whether K-sufficient wheat activated the SA-signalling pathway or not after aphid infestation on the basis that PAL activity significantly increased after aphid infestation. With the presence of crosstalk between the JA and SA-signalling transduction pathway, we speculated that the JA and SA-signalling pathway in K-sufficient wheat were activated at different aphid infestation times, respectively. However, further analysis needed to elucidate. Our results showed that K-deficient wheat susceptibility to aphids might be due to K-deficient wheat could not initiatethe signaling transduction effectively or activate the defense system after aphid infestation.