We report a novel method to prepare graft copolymers (PS-g-PEGM and PS-g-PTHF) with controlled backbone length, grafting sites, and spacing length. In this method, first, RAFT polymerization of styrene (St) was performed by using polytrithiocarbonate as chain transfer agent to produce trithiocarbonate-containing polystyrene (PS); the experimental results show that the chain length of the backbone and PS chain between neighboring trithiocarbonate units can be well-controlled. Second, trithiocarbonate-containing PS reacted with maleic anhydride (MAh) to form MAh-containing PS; thus, grafting points were inserted into the PS backbone between St and trithiocarbonate units. Third, poly(ethylene glycol methyl ether) (PEGM) or poly(tetrahydrofuran) (PTHF) chains were linked onto the main chain by esterification reaction of maleic anhydride with PEGM or PTHF to form graft copolymers (PS-g-PEGM with M-n = 24 900 and M-w/M-n = 2.1; PS-g-THF1 with M-n = 38 000 and M-w/M-n = 1.86; PS-g-PTHF2 with M-n = 56 000 and M-w/M-n = 1.92) with controlled backbone length, grafting sites, and spacing length.