To return to a function that reflects actual nitrogen fertilizer and yield effect，and calculate the appropriate optimal application amount of nitrogen fertilizer through reasonable analysis of marginal cost composition，are the key to the recommendation of nitrogen fertilizer by fertilizer effect function method.It can also provide a reasonable optimal yield for other fertilizer recommendation methods.Based on the 13-year long-term positioning test，the fertilizer nitrogen treatments was set up as N 0，45，90，120，180 and 240 kg/hm².The average yield from 2015 to 2017 was used to establish the nitrogen fertilizer yield effect function to find the optimal recommended amount of nitrogen fertilizer.The marginal cost was calculated including marginal cost of nitrogen fertilizer，marginal cost of absorption of phosphorus by wheat and corn and marginal cost of absorption of potassium by wheat and corn.The nutrient cost of absorbing phosphorus and potassium was equivalent to that of fertilizer phosphorus and potassium.Using the relatively stable yield data of long-term positioning test，the effect equation of nitrogen fertilizer on the yield of wheat and corn was obtained，and the regression coefficient was greater than 0.9946.When marginal production to investment ratio was 2，optimal nitrogen application rate decreased from 207.15 kg/hm² to 196.50 kg/hm²； When marginal production to investment ratio was 5，optimal nitrogen application decreased from 193.54 kg/hm² to 158.55 kg/hm².The recommended amount of nitrogen fertilizer can be reduced by improving the marginal cost calculation method，that is，the improvement the marginal cost calculation method can correct the problem that the current recommended fertilizer amount is too high.