The invention relates to an environment-friendly drilling fluid granularity optimization method, which comprises the following steps of: adding nano-micro particles and micro-fine particles into a drilling fluid at room temperature, and then stirring at a high speed for 30 to 60 minutes, so that the drilling fluid granularity distribution after the particles are added is normal distribution within20 [mu]m, and the whole drilling fluid granularity distribution is bimodal distribution, so as to realize the drilling fluid granularity optimization. The drilling fluid comprises 3-5wt% of sodium bentonite and 1-3wt% of sulfonated phenolic resin, and the pH value of the drilling fluid is 7-10. The particle size range of the nano-micro particles is 2-5 [mu]m, and the content of the nano-micro particles in the drilling fluid is 0.1-5wt%; and the particle size range of the micro-fine particles is 5-20 [mu]m, and the content of the micro-fine particles in the drilling fluid is 0.1-8wt%. According to the method, non-toxic particles with different particle sizes are used for optimizing the particle size distribution of the drilling fluid, high-quality mud cakes are formed on the well wall, safe and rapid drilling is facilitated, and the method is suitable for the fields of petroleum drilling leakage type leaking stoppage, well wall reinforcement and the like.