Abstract: By using the time series data from 1978 to 2014 in China, the vector autoregressive model was established to study the relationship among the urbanization, agricultural modernization and grain output. The results showed that: first, there was a one-way causation between the urbanization and the grain output, and the urbanization was the Granger cause of the fluctuation of grain output. By using the impulse response function and variance analysis, it was indicated that the urbanization had a negative impact on the grain output and the negative impact had a long lasting effect, while the urbanization contributed greatly to the fluctuation of grain output. Second, there was a one-way causation between the urbanization and the agricultural modernization, and the urbanization was the Granger cause of the agricultural modernization. By using the impulse response function and variance analysis, it was indicated that the urbanization had a positive impact on the agricultural modernizations and the positive impact had a long lasting effect, while the urbanization contributed greatly to the fluctuations of agriculture modernization.