The utility model discloses a pretreatment system used before pressurization of a feed gas wellhead. The pretreatment system comprises a high-efficiency slug flow trapping system, a vertical gas-liquid separation system, a hydrocarbon liquid pump system, an emergency liquid discharge system, a monitoring system and a logic control system. The device is based on the control law and design requirements of the slug flow catcher; in combination with the liquid level control characteristics of the gas-liquid separator, a scheme of combining the micro-effusion slug flow catcher and the vertical gas-liquid separator is provided from the perspectives of improving the use efficiency of the slug flow catcher and optimizing system control, and the slug flow catching capacity of the slug flow catcheris reserved to the maximum extent; an electric pump process is optimized, and the control level and the gas-liquid uniformity in a downstream medium are improved; and a quick release function is set,so that the adaptability of the system to the large-breadth slug flow working condition is expanded.