To quickly predict and evaluate the effects of turbulence and thermal blooming on the beam spreading during high-energy laser (HEL) propagation in atmosphere, this study examined the scaling law for the beam spreading during laser propagation in atmosphere. By calculating the variation law of the simulated beam spreading with characteristic parameters of atmosphere and laser parameters, the paper built a scaling relationship for the beam spreading during laser propagation in atmosphere with both the turbulence effect and the thermal blooming effect. This scaling relationship reveals that when the initial beam quality of the laser is poor, the beam spreading caused by diffraction and turbulence grows with the ratio of the aperture of the transmitting system to the atmospheric coherence length at a faster rate. Also, the effect of thermal blooming on beam spreading intensifies as the Fresnel number of the transmitting system increases. The established scaling relationship performs well in rapidly predicting and evaluating the beam spreading laws of lasers with different powers under turbulence and thermal blooming effects of different intensities. Therefore, it can provide a reference for the design and optimization of laser systems.