The invention provides an optical measurement method of a strain rate field in a plane based on velocity linear fitting, and belongs to the technical field of object strain rate field measurements. The optical measurement method comprises the following steps: collecting a plurality of images of an objected to be measured in a loading process, acquiring measuring point coordinates of triangle pixel blocks in the images, acquiring velocity fields of measuring points according to interval times of the images, acquiring fitting data points of each triangle pixel block according to the coordinate and velocity of each measuring point in each triangle pixel block, thus acquiring a coefficient of a fitting plane of each triangle pixel block, finally, acquiring the strain rate field of the object to be measured in the plane according to the obtained relationship of the coefficient and a velocity partial derivative. The optical measurement method disclosed by the invention is based on ideas of data fitting, better than the conventional correlation method, simple in process, convenient and fast in steps and easy to understand, and results can be obtained only by solving coefficients of fitting plane equations. The optical measurement method considers information of internal measuring points of discrete pixel blocks to improve the accuracy and the reliability of final results and increase the flexibility in selection of the measuring points.