The invention provides a method for preparing high carbon hydrocarbon through photocatalytic activation of biomethane. The method comprises the steps that a biomethane CO2 removing system, an air pump, a preheater and multiple stages of parallel photocatalytic activation reactors are included, and unconverted methane mixture gas is used circularly and repeatedly; hydrogen generated in a photocatalytic activation reaction is permeated through a palladium hydrogen permeation membrane separator and recycled through a hydrogen vacuum pump, methane, ethane, ethylene, propane and butane are impermeable, high carbon hydrocarbon is recycled through a condensation cooler and a gas separator, uncondensed gas and methane are merged and return to a reaction system. According to the method, the hydrogen byproduct can be recycled, due to the fact that a multi-stage parallel photocatalytic activation inducing reaction is adopted, the staying time of membrane is prolonged, the conversion rate of membrane is increased, generation of ethane, ethylene, propane and butane in the reaction system is promoted, the alkanes serve as a reaction auxiliary of membrane and an initiator of optical free radical chains to be used circularly and repeatedly, therefore, the temperature of the reaction for converting membrane into high carbon hydrocarbon is significantly lowered, energy resources are saved, and the production cost of high carbon hydrocarbon is reduced.