The invention provides a novel process for preparing p-phenylene diisocyanate (PPDI) based on phosgene, which relates to a process for preparing chemical raw materials, in particular to a novel process for preparing the p-phenylene diisocyanate (PPDI). The process comprises the following steps: 1, hydrochloric acid formation reaction; 2, salifying reaction; and 3, photochemical reaction which is to transfer a mixture solution of para-phenylene diamine dihydrochloride-o-dichlorohenzene to a photochemical kettle, control the temperature, introduce the phosgene in an infinite reflux state to react, determine that the reaction is terminated after reaction materials are settled, introduce quantitative nitrogen to remove acid, sample and analyze the reactant, and obtain the p-phenylene diisocyanate PPDI through rectifying, crystallizing, centrifugating, drying, packaging or refining, slicing and packaging when the acidity content is qualified. In the process, a reasonable salifying reactioncondition is customized to avoid slow reaction of a prepolymer, complicated reaction mechanism, and easy generation of side products and adverse factors influencing the reaction in the next step.