Static stiffness is an important safety index of hoisting machinery， and its main measurement parameter is the deflection value of the crane girder in the case of no-load and specified load. Before and after loading， the gray features of the bottom boundary of the beam remain stable， and the displacement of the beam boundary in the image plane can be accurately calculated by using the digital speckle correlation algorithm. When the parameters of the optical imaging system are known， the actual displacement of the beam can be accurately calculated from the displacement in the image plane. Combined with the characteristics of oblique axis imaging， the static stiffness measurement system is built， and the measurement error of the oblique axis optical system is analyzed， and the selection of sub region location and sub region size is optimized. The system is used to measure the movement of the simulated girder， and the error is within 0.2 mm. The static stiffness of the crane girder is measured at a distance of 30 m， and the measurement results are consistent with those of the control method， which indicates that the method could meet the requirements of the static stiffness measurement of the crane girder.