The invention discloses a separation membrane pore diameter determination method. The separation membrane pore diameter determination method comprises the following steps: (1) cutting a separation membrane to be detected into small sections and putting the sections into container filled with infiltration liquid; putting the container into a vacuumizing device and vacuumizing until membrane pores of the separation membrane are full with the infiltration liquid; (2) taking out the sections of the separation membrane with the membrane pores which are full with the infiltration liquid out from the infiltration liquid; putting the sections into a density meter filled with the infiltration liquid and measuring the volume V; (3) taking out the separation membrane sections with the measured volume, wiping the residual infiltration liquid on the surface of the separation membrane and putting the separation membrane into a glass sample bottle; (4) putting the glass sample bottle into a glass test pipe and putting the glass test pipe into a low-field nuclear resonance spectrometer and starting to test; and carrying out inversion calculation to obtain the porosity, the pore diameter size and the pore diameter distribution of the detected separation membrane. The separation membrane pore diameter determination method provided by the invention is simple to operate and high in accuracy.