Firstly, the molecular coil size of 4 kinds of polymers, whose molecular weight polymers is 8 million, 12 million, 16 million and 20 million respectively, was measured by the light blockage counter. Then pore cumulative distribution curve of field cores was measured by kerosene mass method. Finally the relationship curves between polymer molecular coil size and inaccessible pore volume of field cores were plotted. On the basis of this study, the water saturation value is assigned to the volume of the pore volume, and the polymer molecular weight with better compatibility with rock pores is optimized according to the relationship curve drawn. This method is based on the micro channel of underground rock. It has reasonable value and simple steps. And there are some guidances for the selection of the polymer in the oil field.