Analyzed the iso-pressure approximate model used in laser propagation in gas medium and pointed out its disadvantage under complex border conditions. The weakly compressible fluid model was developed which was based on the Mach uniform method introduced by Peter Wesseling. Using the weakly compressible fluid model and scalar wave equation, a numerical study was carried out for the gas thermal effects on laser propagating in a closed tube full-filled with air. The far-field beam shape at different time were obtained and compared with the experiment ones. It is shown that the weakly compressible fluid model could be well adapted for problems with complex border conditions, acute than the compressible fluid model or the iso-pressure approximate model.