The research demonstrates that when the temperature changes a lot, the internal components of the large sky area multi-object fiber spectroscopic telescope (LAMOST) spectrographs will produce thermal-induced flexure, which will cause the image blurring of the CCD. Such image blurring reduces the measurement accuracy of astronomical data and greatly increases the difficulty of the daily maintenance of the 16 spectrographs.Therefore, a defocus diagnosis method for multi-object image definition evaluation is proposed based on the calibration lamp spectrum taken by LAMOST. By analyzing the spectrum image under the different defocusing amounts, the half-height full-width (FWHM) and its distribution of spots are obtained to establish the function model between the multi-target image definition and the defocusing amount of the system. It can realize the fast defocus diagnosis of the spectrum image quality of LAMOST and provide a reference for the realization of intelligent active compensation technology of LAMOST spectrograph. In this paper, the principle and structure of the LAMOST spectrograph are introduced, and the demand for the focusing precision of the spectrograph is given. And the measurement principle of FWHM value and the construction method of the multi-object image definition evaluation function are introduced in detail. Compared with the traditional image definition evaluation function, the results show that the proposed method has a higher definition ratio and accuracy. And the defocus diagnosis error of the spectral image is less than 10 μm, which effectively reduces the error caused by artificial diagnosis and improves the consistency of the 16 spectrographs. This method can also improve the daily operation efficiency and long-term stability of the LAMOST spectrographs.