A horizontal gas-liquid separator for an air conditioner includes a housing and a refrigerant inlet pipe. The housing defines a cavity. The cavity has a gas outlet formed in the top of the cavity and a liquid outlet formed in the bottom of the cavity. A minimum distance between the gas outlet and a left sidewall of the cavity is denoted by L1, and a minimum distance between the liquid outlet and the left sidewall of the cavity is denoted by L2. The refrigerant inlet pipe is located on a left side wall of the housing and has an end extending into the cavity. A distance between an end face of the end of the refrigerant inlet pipe and the left sidewall of the cavity is denoted by L3, and L3≥L1, L3≥L2.