Based on the survey data of agricultural machinery purchase of small and medium-sized farmers in Weifang, Shandong Province in 2006 and 2018, respectively, the influence of different periods of agricultural machinery purchase subsidies, characteristics of agricultural machinery purchase decision makers and family characteristics on farmers’ purchase intention was calculated by using Logit model. The results showed that with the passage of time, the purchasing behavior of agricultural machinery tended to be rational, and the efficiency of agricultural machinery purchase subsidy was greatly reduced. Keeping other conditions unchanged, the purchase subsidy of agricultural machinery increased by 1%, the probability ratio of farmers’ purchase was increased by 9.04% in 2006, and decreased to 2.27% in 2018;the income of farmers increased by 1%, the probability ratio of purchasing machines was increased by 0.46% in 2006, and decreased to 0.13% in 2018;the income of farmers’ planting industry increased by 1%, the probability ratio of farmers’ purchase was increased by 0.29% in 2006, and decreased to 0.08% in 2018;the cultivated land area of farmers increased by 1%, the probability ratio of purchasing machines was increased by 0.33% in 2006, and decreased to 0.02% in 2018. According to the calculation results, some policy suggestions were put forward, such as meeting the needs of farmers, improving the efficiency of subsidies, increasing the types of subsidies, and reasonably determining the amount of subsidies.