NOVELTY - Separating propylene and propane from a liquid hydrocarbon, comprises measuring if molar content ratio of a liquid hydrocarbon containing carbon two atom to propylene is less than 0.004, and than passing the liquid hydrocarbon into a depropanization tower and a propylene rectification tower with double-tower gas fractionation device to separate the high purity propylene and propane from the qualified liquid hydrocarbon.    USE - The method is useful for separating propylene and propane from a liquid hydrocarbon (claimed).    ADVANTAGE - The method separates propane and propylene with high yield and good applicability in a simple, short, cost-effective and environmental friendly manner. The propane and propylene are easily popularized in industry.    DETAILED DESCRIPTION - Separating propylene and propane from a liquid hydrocarbon, comprises measuring if molar content ratio of a liquid hydrocarbon containing carbon two atom to propylene is less than 0.004, than passing the liquid hydrocarbon into a depropanization tower and a propylene rectification tower with double-tower gas fractionation device to separate the high purity propylene and propane from the qualified liquid hydrocarbon, condensing the top of the gas phase using the condenser, passing a part of the condensed liquid phase into the depropanization tower through the first reflux pump to rectifying, passing another part into the propylene rectifying tower, passing a part of the propane removing tower liquid phase to a reboiler through the first tower reflux pump, then heating to a certain temperature and passing into the depropanization tower, passing another part of propane removing tower liquid phase into the mixed product output device, providing heat source to the rectifying tower, balancing carbon three atom using control tower, reverse flowing the tower top gas phase into a return tank through a condenser, then through the second reflux pump to the propylene rectification tower, and passing another part of the liquid phase into a second reboiler through a tower reflux pump to obtain output product, where: the double-tower gas fractionation device is provided with a feeding port, which is connected with the depropanization tower for extracting liquid hydrocarbon through a liquid hydrocarbon pump from liquid hydrocarbon tank; the top part of the return tank is provided with a first condenser, a reflux tank and a reflux pump; the bottom part of reflux tank is provided with a reboiler and a tower reflux pump; and the top part of propylene rectification tower is provided with a second condenser, a second reflux tank and a reflux pump.