A water chilling unit liquid storage horizontal gas-liquid separator comprises a horizontal drum, a front cover, a rear cover, an air outlet pipe and three liquid observing devices. The horizontal drum is a horizontally-arranged circular iron drum. The top of the horizontal drum is provided with a safety valve and a refrigerant filling valve. The front cover is formed by welding an arc-shaped spherical surface plate with a cylindrical shell. The rear cover is formed by welding an arc-shaped spherical surface plate with a cylindrical shell. The air outlet pipe is a bent pipe of which the two ends use straight pipe bodies and the middle is in smooth transition. Each liquid observing device comprises an internal thread sleeve and an observation window. Each observation window comprises a lensframe and a lens. According to the gas-liquid separator, the refrigerant filling valve is used direct refrigerant supplementation when refrigerants need to be supplemented, a liquid return device isused for releasing the refrigerants when the refrigerants need to be taken back actively, and safety operation of the gas-liquid separator is ensured.