Abstract: Based on Busch hole model combined with Fick's second law, we built the mathematical model of low rank coal loss volume, and revised USBM gas content testing method. According to the testing data of low rank coal in Fukang Mining Area, we calculated or tested gas content by modified model, USBM method and adsorption isotherm inversion method. The results show that the calculated loss gas volume by USBM method is not accurate; the calculated result is lower than 1 m³/t to 3.5 m³/t of modified model or the isothermal adsorption inversion. The calculated results of gas content by the modified and the results by the isothermal adsorption inversion are similar, which can reflect the actual gas content of the low rank coal.