Abstract: To improve the gas-filling efficiency by compressor in compressed natural gas (CNG) conventional fueling station, the start-up conditions and gas-filling performance under the mode that the compressor directly fills the vehicle CNG cylinder were investigated. The process calculation model was established with Hysys for CNG conventional fueling station. Then, marginal and initial conditions were set for the model depending on actual capacity and operation environment of the station to dynamically simulate the process in the station. Some key variables of the compressors, such as energy consumption, operating time and start-up frequency in a day and in peak time, were obtained. Besides, a new process parameter – gas filling volume per unit of energy consumption was proposed to characterize the economic performance of the compressor under different working conditions. Research results show that higher gas-filling efficiency of compressors can be observed in direct filling mode. With the increase in start-up pressure of the compressor under direct filling mode, both gas filling volume per unit of energy consumption and the minimum pressure in fully filled vehicle CNG cylinder may increase.