Standard of payment is the core content of a payment for ecosystem services (PES) project. Considering the conversion of farmland to a variety of forestlands, based on the definition of the PES standard range, we calculated the increments of ecosystem services in the Weihe River Basin in Gansu Province with the InVEST model and the weighted method, using the land use, meteorological, soil, socioeconomic, and questionnaire survey data, to improve operability of the payment standard. The difference between forestland and farmland in ecosystem services was used to characterize the main increments of ecosystem services of water yield, sediment retention, and water purification. The number of ecosystem services for forestland came from the weighted sum of weights (the ratio of each kind of converted forest area accounted for the total conversion area) and the number of ecosystem services per unit area of corresponding forest. Additionally, setting the increased value of ecosystem services as the payment upper limit and taking the opportunity cost of participators as the lower limit, the differentiated PES standard of returning farmland to forest in 15°-25° and above 25° sloping farmland were studied from the standpoint of a new round of national policy oriented towards returning farmland to forest. Furthermore, from the standpoint of fair distribution of benefits, the differentiated PES standard in different net income regions, coupled with opportunity cost input and ecosystem services output, were explored. The results showed that the PES standard of returning farmland to forest was range from 146.39×10⁴ Yuan km^-2 a^-1 to 481.98×10⁴ Yuan km^-2 a^-1 in the Weihe River Basin of Gansu Province. With returning farmland to forest, the annual value of water yield would increase by 1.49×10⁴ Yuan/km²; the annual value of sediment retention would increase by 479.55×10⁴ Yuan/km²; and the annual value of water purification would increase by 0.94×10⁴ Yuan/km². Secondly, the results showed that the areas in sloping farmland (15°-25°), which were the upstream faces of important drinking water sources in first-level protected and reserved areas, accounted for approximately 10% of the sloping farmland. If all above-mentioned areas in sloping farmland (15°-25°) changed to the forestland, PES funds of 13.95×10⁸ Yuan to 33.55×10⁸ Yuan would be needed. For those above 25°, PES funds ranging 12.96×10⁸ Yuan to 28.35×10⁸ Yuan would be needed. For the areas in sloping farmland (15°-25°), the increased annual values of water yield, sediment retention, and water purification were 13.35×10⁴, 337.42×10⁴, and 0.81×10⁴ Yuan/km², respectively. Values for the areas above 25° were 15.99×10⁴, 302.90×10⁴, and 0.76×10⁴ Yuan/km², respectively. The ranges of PES for the areas in sloping farmland (15°-25° and above 25°) were 146.39×10⁴ to 351.58×10⁴ Yuan km^-2 a^-1 and 146.39×10⁴ to 319.65×10⁴ Yuan km^-2 a^-1. Finally, compared with other districts, the maximum net income of returning farmland to forest appeared in the Maiji district, and the minimum was represented in the Longxi district. Their ranges of PES were 202.23×10⁴ to 727.92×10⁴ Yuan km^-2 a^-1 and 96.99×10⁴ to 136.53×10⁴ Yuan km^-2 a^-1, respectively.