The invention relates to a fine velocity modeling method based on gas cloud region constraints. The method comprises steps: a velocity model comprising a three-dimensional data mesh is defined, to-be-detected space is divided into a gas cloud region and a non-gas cloud region, and velocity data in the known logging data are used for counting velocity change trends of the gas cloud region and the non-gas cloud region; seismic interpretation horizon data are used and gas cloud region space distribution is combined to carry out block processing on the velocity model; velocity models for the gas cloud region and the non-gas cloud region are built; and a seismic interpretation horizon control technique is adopted to combine the velocity models for the gas cloud region and the non-gas cloud region, and an initial velocity model is acquired. In view of special geological phenomena in the gas cloud region, the gas cloud region and the non-gas cloud region are effectively distinguished, block modeling is carried out on the gas cloud region and the non-gas cloud region respectively, and through research on the velocity change trends of the gas cloud region and the non-gas cloud region, velocity modeling is carried out in a more targeted mode, the actual geological conditions are met, and the time-depth conversion precision is high.