Optical detection and ranging techniques based on range-gated active imaging systems have been greatly developed. Flash lidar obtains slice images of targets by accurately measuring the time of flight of laser pulses, and effectively reduces the influence of backscattering on imaging results through range gating technology. It has the ability to work in the harsh environments. Therefore, flash lidar is widely used in various fields. However, in the flash lidar daytime measurement, since sunlight dominates the backscattered noise, when the backscattered sunlight is received by the lidar detector, there is a huge amount of energy due to the wide wavelength range of solar radiation. The huge energy will make the target mixed with the sunlight. Even if the range gating technology is used, the influence of sunlight on the intensity image received by the lidar cannot be avoided. As a result, the flash lidar cannot obtain target distance information during the daytime, severely limiting the application of flash lidar. In order to eliminate the light pollution of sunlight on the target, this paper adopts the intensity image preprocessing method based on morphological opening operation to preprocess the original intensity image received by the flash lidar during the day, and then uses the adjacent frame difference method to get the distance information of the target. First of all, a structural element is selected according to the principle of selecting structural elements for morphological opening operation. Secondly, the chosen structural element is used to preprocess the target slice images received by the flash lidar imaging system using morphological opening operation, while the preprocessing results are evaluated by average intensity image. Thirdly, the adjacent frame difference method is used to obtain the distance information of the target from these preprocessed intensity images. In order to verify the feasibility of this method, we use a self-developed flash lidar to conduct imaging experiments on a target building at a distance of 500 m during the daytime. The raw average intensity image obtained from the raw slice image received by the flash lidar system shows that the sunlight scattered by the surrounding target is mixed with the laser echo signal reflected back from the target building, making it difficult to distinguish the target building. Therefore, we can only obtain the distance information of the target by preprocessing the raw slice image obtained by the flash lidar system. Based on the principle of preprocessing the original intensity images with morphological opening operation, in this experiment, a $\mathrm{3}\times \mathrm{3}$ cross-shaped structural element is selected by the structural element selection principle. The chosen structural element is then used to preprocess the raw sliced images of the building target at 500 m received by the flash lidar system with morphological opening operation, while compared with binarized average intensity image from Gaussian filtering preprocessing, median filtering preprocessing and threshold segmentation preprocessing. The results of the comparison amply demonstrate that preprocessing based on morphological opening operations can extract the target from sunlight pollution. After that, the adjacent frame difference method is used to obtain the distance information of the target from the preprocessed intensity images based on these four methods. Using structural similarity to evaluate the distance information of the reconstructed target building, the morphological opening operation preprocessing is compared with Gaussian filtering preprocessing, median filtering preprocessing and threshold segmentation preprocessing, and the structural similarity is 0.78, 0.83, and 0.85, respectively. By analyzing the structural similarity between them, the result fully demonstrated that the preprocessing method using the morphological opening operation could completely obtain the distance information of the target while removing the isolated noise caused by sunlight.