Active illumination with narrow spectrum laser based on ladderlike phase modulator (LPM), is an effective method to improve the illumination uniformity. Laser spectrum style, lens number of LPM, and lens thickness error of LPM are three significant factors influencing the illumination uniformity in engineering. The laws of the three factors with scintillation indexes are analyzed through numerical simulation. The results indicate that the laser spectrum style is not an important factor, and the spatial scintillation index of uniformity, Gauss, Lorenz style narrow spectrum laser are 0.27, 0.28, and 0.28 respectively. The scintillation indexes decrease gradually with the lens number of LPM increasing. The spatial scintillation index increases from 0.266 to 0.271, when the lens thickness error of LPM is –0.6 mm. Therefore, several ideas are proposed: when this method is used in the engineering project, some attention should be paid to laser spectrum style, the lens number of LPM should be optimized by considering the laser spectrum width and scintillation index synchronously, and the negative error of optical lens thickness gradient of LPM should be controlled attentively.