Shale gas is produced by gas transport under constant reservoir temperature and down hole pressure conditions. Therefore, it is of great importance to study the dynamic gas adsorption/desorption-diffusion process in shale, under isothermal and constant production pressure conditions. Accordingly, a new experimental method and apparatus has been designed and tested for studying shale gas transport behavior. The essence of the method includes accurately measuring the gas going into or coming out of a shale sample with respect to time. The accuracy and sensitivity of the method are confirmed by conducting experiments with methane and helium, and comparing the outcomes from adsorption isotherm obtained using the traditional constant-volume method. With this newly designed method, a two-stage transport process was observed by comparing the dynamic gas transport of N-2 and CH4. Free gas transports first due to the pressure gradient, which is followed by the desorption and transportation of the adsorbed gas. Besides, tests under five pressures were conducted. It is found that for the same differential pressure, higher external pressure could accelerate the process while decrease the amount of transported gas. Finally, the dynamic adsorption diffusion (DAD) mathematical model is presented to analyze the gas transport mechanisms in shale depicting the adsorption/desorption diffusion process under isothermal and constant external pressure. By calculating the production rate for free gas and adsorbed gas, the two stages of the transport process can be identified. This study provides a straightforward method to experimentally determine the dynamic gas adsorption/desorption diffusion process in shale, which is a relatively simple but information rich technique for the assessment of shale gas targets.