An explicit relation between the Bragg shift of the grating readout angle and the shrinkage of photopolymer samples is presented based on the changes in amplitude and direction of the grating vector, caused by lateral shrinkage and thickness shrinkage of photopolymer after exposure. A two-step experimental method to measure the Bragg shift of the unslanted grating and slant grating in photo polymer separately is designed and implemented with blue light readin and red light readout. The lateral shrinkage and thickness shrinkage of two self-made blue-light photopolymer samples are calculated respectively, and their volume shrinkage are obtained. The results show that the volume shrinkage of the sample with modified composition is less than 0.5%. This study also shows that the lateral shrinkage of photopolymer is generally not negligible. By using this method, it is easier to measure the shrinkage of a large amount of photopolymer samples, and find the relation between shrinkage and compositions of photopolymer.