Disclosed is a method for optimizing a particle size of controlled-release nitrogen fertilizer oxamide particles for single basal fertilization, which comprises the following steps: measuring a pH value of the soil in a region; measuring an organic matter content OM of the soil, with a unit of g·kg−1; measuring a total nitrogen content TN of the soil, with a unit of g·kg−1; collecting an average temperature T at a 10 cm depth of the soil in a rice growing season in the region, with a unit of ° C.; and for a rice variety with a growth duration of D, with a unit of day, calculating a diameter of controlled-release nitrogen fertilizer oxamide particles for single basal fertilization that matches a nitrogen requirement in the rice growth duration by using a formula.