The invention provides a synchronous telescopic mechanism of a truck-mounted crane. The synchronous telescopic mechanism comprises a basic arm, a first telescopic arm, a second telescopic arm, a thirdtelescopic arm, a multi-stage telescopic cylinder and a rope row mechanism, and the first telescopic arm, the second telescopic arm, the third telescopic arm, the multi-stage telescopic cylinder andthe rope row mechanism are sequentially connected into the basic arm in a sleeved manner; the rope row mechanism includes a first arm stretching rope assembly, a second arm stretching rope assembly, afirst arm shrinking rope assembly and a second arm shrinking rope assembly; the multi-stage telescopic cylinder includes a piston rod, a first-stage cylinder barrel, a two-stage cylinder barrel and abracket arranged at the end, far away from the first-stage cylinder barrel, of the two-stage cylinder barrel; and the first arm stretching rope assembly comprises a first arm stretching wheel and a first arm stretching rope, the second arm stretching rope assembly comprises a second arm stretching wheel and a second arm stretching rope, the first arm shrinking rope assembly comprises a first armshrinking wheel and a first arm shrinking rope, and the second arm shrinking rope assembly comprises a second arm shrinking wheel and a second arm shrinking rope. The synchronous telescopic mechanismhas the characteristics of being simple in structure and convenient to install.