He decided to turn his back on his family and start a new life.

turn 在这里是动词，意思是"转身、转向"。 his 是形容词性物主代词，表示"他的"。 back 是名词，指"背部、后面"。 on 是介词，表示"在...上"或"朝向"。 turn one's back on 是一个固定短语，字面意思是"把背转向某人"，引申为"背弃、抛弃"的意思。