Thermal blooming effect of inner optical path remarkably affects far-field beam quality and energy distributions which should be taken into account in high energy laser (HEL) system. A physical model of thermal blooming is established. Based on the model, numerical simulations are carried out to study both the influences of absorptions of laser energy and tube structures on laser propagation in a closed tube. The natural convection of gas is numerically simulated by computational fluid dynamics (CFD) method. Gas temperature distributions, additional phase differences (APDs), variations of beam quality and drifts of mass center in far-field under different absorptions of laser energy and tube structures (Z-shaped and U-shaped) are compared, respectively. By analysis of numerical simulation results, the switch time of heat conduction and heat convection in gas is distinguished, which significantly affects the variations of beam quality and drifts of mass center in far-field. In addition, it also indicates that less absorption of laser energy improves beam quality and delays the switch time of beam quality between two heat transfer mechanisms. Therefore, it is significant to control the absorptions of laser energy for HEL system in practice. Different tube structures owning different beam paths change the distributions of APDs and thus influence beam quality. APDs of the two horizontal sections are the same (superposition effect) for Z-shaped tube while inverse (compensation effect) for U-shaped tube. It is shown that drifts of mass center in far-field are greatly suppressed for U-shaped tube than that of Z-shaped tube and beam quality is also improved.