A dynamic model of gas turbine, which takes both before-turbine and post-turbine capacity into consideration, is developed. The dynamic behavior of gas turbines in start-up by high-pressure gas is calculated by using the model, and its principal influence factors are analyzed. The calculated results show that the before-turbine capacity has notable influence on the transients of both entrance and exit pressures of the gas turbines, the post-turbine capacity mostly affects the dynamic processes of the turbines' exit pressures, and has little influence on the turbines' entrance pressures.