The measurement accuracy of telecentric imaging technique straightforwardly depends on the calibration of the telecentric camera. We present a flexible and simple calibration method based on a telecentric imaging model and orthogonality of rotation matrix. First, we use the orthogonality of rotation matrix to solve the magnification. Second, the external parameters are solved by the imaging model, and the ambiguity of sign is solved. Finally, we use the LM nonlinear optimization method to solve the distortion parameters. Experimental results show that the reprojection error is 0.7 pixels, which represents the actual dimension of 6.37 µm. In addition, the standard measuring block and real objects are measured, and the results verify the measurement accuracy and robustness of the proposed method.