Homologous dual-light-path visibility instrument is a visibility instrument based on digital camera method. The brightness of the light source obtained by the CCD sensor is an important part of the spot extraction algorithm and the visibility inversion model. The background light noise seriouly affects accuracy of visibility inversion. Aiming at the large error of visibility inversion under complex background light disturbance, we use the amplitude modulation method to redesign the modulated light source, and demodulate the CCD image so that the background light is eliminated effectively. The experimental results show that the light source amplitude modulation method can effectively remove the background light, the random noise, and the dark noise of the camera. When the modulation points are within 32 to 128, the image denoising effect is good and the peak signal-noise-ratio (PSNR) of the speckle image after denoising is greater than 30. In this modulation point range, the larger the number of modulation is, the larger the PSNR is and the smaller the root mean square error is. Inversion accuracy of the visibility is greatly improved in the range of the above modulation points, which means that the amplitude modulation method can significantly improve the accuracy of visibility inversion.