The invention relates to a car roof rack, in particular to a car roof rack mounting clearance compensation structure. The clearance compensation structure includes a top cover outer plate, an arc-shaped reinforcement plate, a roof rack supporting base and a clearance compensation device. The upper end of the clearance compensation device is fixed to the roof rack supporting base, and the lower endis fixed to the arc-shaped reinforcing plate. The clearance compensation device is arranged between the arc-shaped reinforcing plate and the roof rack supporting base, force on the roof rack supporting base is directly transmitted to the arc-shaped reinforcing plate through the clearance compensation device, bearing reliability is good, and the top cover outer plate is well protected. The clearance compensation device adopts a structure of an outer sleeve and an inner sleeve, and the outer sleeve and the inner sleeve are connected by threads, so that free adjustment of the clearance compensation device in the height direction is achieved, and the clearance compensation device can be applied to various types of cars. A mounting bracket is arranged at the upper end of the outer sleeve, elastic clamping legs and positioning flanges of the mounting bracket are matched with rectangular mounting holes, so that the clearance compensation device is extremely easy to install and disassemble.