The estimation of lost gas during borehole coal sampling determines the accuracy of coalbed methane (CBM) content determination. The isothermal desorption data is used frequently for analyzing the estimation error of the lost gas, which is not consistent with the engineering practice that the gas desorption during borehole coal sampling is in dynamic temperature changing condition (i.e., DTC), thus resulting in the result of the lost gas estimation is not accurate. In this study, a DTC adsorption/desorption experimental system was set up to study the gas desorption characteristics of coal particles during borehole coal sampling, and the differences between isothermal desorption and DTC desorption were analyzed in detail. Furthermore, the isothermal desorption data and DTC desorption data were fitted using the unipore diffusion model (UDM) to discuss the influence of DTC desorption on the lost gas estimation. Finally, the fractional diffusion model (FDM) was applied to estimate the lost gas. The results showed that there is a significant difference between isothermal desorption and DTC desorption. For the case of DTC desorption, more gas will be lost at the same exposure time, resulting in the lost gas being underestimated in the CBM content determination. With the exposure time of 12 min, the average fitting errors of lost gas using the FDM with different coal particle sizes and gas pressures are less than 17.9%, while that using the UDM are greater than 57.5%, suggesting that the FDM could improve the fitting accuracy of lost gas estimation in the CBM content determination.