The invention discloses a horizontal gas-liquid separator behind an evaporator for a cooling water unit. The gas-liquid separator comprises a horizontal cylinder, a front cover, a rear cover, an air outlet pipe and visual liquid level devices; the horizontal cylinder is a horizontally-placed circular iron cylinder, and an outlet of an inlet pipe is welded with a transverse plate; the front cover is formed by welding an arc-shaped spherical panel and a cylindrical shell; the rear cover is formed by welding an arc-shaped spherical panel and a cylindrical shell; the air outlet pipe is the pipe with straight pipes at the two ends and with a smooth transition bending middle; and the number of the visual liquid level devices is three, the visual liquid level devices comprise inner threaded sleeves and observation windows, and the observation windows comprise lens frames and lenses. According to the gas-liquid separator, the inlet pipe is arranged to enable a refrigerant entering the horizontal cylinder to be far away from the air outlet pipe as far as possible so as to increase the gas-liquid separation distance, meanwhile, the transverse plate can play the role of blocking liquid in therefrigerant, so that gas-liquid separation is accelerated.