The invention relates to an optical measuring method of a strain rate field in the plane of an object and belongs to the object strain rate field measurement technical field. The method includes the following steps that: a photographing device is utilized to photograph a plurality of images of one plane of a measured object in a loaded process; measurement images are selected, and the number of the measurement images, sub area size and the number of measuring points are set; the measuring points are dispersed in an image area so as to form non-overlapping and gapless triangular pixel blocks; and a strain rate field in the plane of the measured object is obtained by using Gauss theorem and a geometric equation. The method of the invention is superior to a central difference method-based digital image correlation method. The calculation process of the method of the invention is simple and convenient, only summation calculation is required, so that a result can be obtained; the implementation of programming is easy; and accuracy and reliability can be improved. The optical measuring method has a broad application prospect in the solid experimental mechanics field.