The invention discloses a polyurethane photochromic lens and a preparation method thereof. The lens comprises the raw materials: polyester diol, isophorone diisocyanate, 2,2-dimethylol propionic acid,methyl methacrylate, styrene, an initiator, glycerol, 3-mercaptopropionic acid, methylbenzene, p-toluenesulfonic acid, a photochromic material and a UV adsorbent. The preparation method comprises thesteps: firstly, mixing glycerol, 3-mercaptopropionic acid, methylbenzene and p-toluenesulfonic acid to carry out a reaction to prepare a curing agent; then, adding polyester diol, isophorone diisocyanate and 2,2-dimethylol propionic acid into a mixed solution of methyl methacrylate and styrene, carrying out mixing, then, adding the curing agent, the photochromic material and the UV adsorbent, andcarrying out stirring to prepare mixed slurry; and pouring the mixed slurry into a die, and placing the die into an air curing furnace to carry out primary curing and secondary curing to obtain the polyurethane photochromic lens. The prepared lens has excellent photochromic and mechanical properties and can be widely applied.