The present invention relates to the technical field of biology, and in particular to a block copolymer and a synthesis method thereof, and a preparation method of nano particles. The block copolymer provided by invention comprises a first block, a second block and a third block, which are connected successively. The first block and the third block are hydrophilic blocks, and the second block is a hydrophobic block. Compared with prior art, the block copolymer provided by the invention can effectively shield surface charges of cationic particles like polycation gene complex particles, exclude in vivo circulation barriers of cationic particles, and improve in vivo circulation efficiency, and also can be grafted to a target group to realize pathological cell targeting in vivo, and effectively improve the targeting effect.