In order to use the camera for non-contact measurement with high accuracy, the monocular camera is usually used for optical calibration, imaging, calibration and measurement, on the basis of the linear model, but there is no analysis and research on the reliability of the commonly used linear model. Thus its reliability is studied and analyzed here. According to the characteristic of the monocular camera, and in order to build the camera model based on relationship of the image pixel coordinates and the three-dimensional world coordinates, the vision measurement experiment is completed by using our own custom template. Then, we get image pixel coordinates by the sub-pixel corner detection method. Neglecting the distortion, the feature extraction and linearity analysis are realized. Many repeated experiments are operated by constantly adjusting the measurement distance. The relative error of measurement by the linear model is less than 1%, and the measurement error is not more than 0.3 mm. Meanwhile, it is found that the model has measurement difference for different regions and object distances. The experiment results show this linear model is simple and practical, and has good linearity within a certain object distance range. These experiment results provide a powerful basis for establishment of the linear model. The work has the potential in practical engineering.