The invention discloses a method for recovering light hydrocarbon and hydrogen from tail gas of a petrochemical plant. According to the method, firstly, the tail gas of the petrochemical plant is compressed and then condensed, and meanwhile condensed hydrocarbon substances are recovered; then, pressure swing adsorption is carried out on the remaining tail gas, and the hydrogen is recovered; afterwards, desorbed gas obtained after pressure swing adsorption is pressurized and condensed, meanwhile, condensed hydrocarbon substances are recovered, and the remaining tail gas enters a gas pipe network to be burned; and then, the hydrocarbon substances recovered two times enter a rectifying tower in a mixed mode to be separated, and therefore the C3 light hydrocarbon and the C4 light hydrocarbon are separated out. According to the method, the recovery rate of the hydrogen is higher than 88%, the recovery rate of the light hydrocarbon is higher than 90%, the purity of collected C3 is higher than 92 mol%, and the purity of collected C4 is higher than 99 mol%.