The invention introduces a thickness control compensation method of a strip steel head part when a welding line passes through a rack, which comprises the following steps: 1, collecting actual rolling parameter values from a process control system; 2, calculating the total adjustment amount self-adaption value of roll-gaps of a single rack; 3, determining the required adjustment time n of the roll-gap according to the roll force of the single rack; 4, setting j to be equal to 1; 5, calculating thickness deviation delta hj-1, delta hj and influencing coefficient factor of the inlet and the outlet of the strip steel according to the thickness of the strip steel at the inlet and the outlet of a rolling mill; 6, calculating rolling force deviation value according to the thickness deviation of the inlet and outlet of the strip steel; 7, calculating roll-gap adjustment deviation value of each time; 8, starting the self-adaptation function of a model; 9, starting calculating function, and dynamically setting roll-gap self-adaptation coefficients in a roll-gap control model by combing the total adjustment quantity self-adaptation value of the roll-gap in the step 2; and 10, carrying out the step 5 when j is less than n and j=j+1; and finishing the process when j is more than or equal to n. The method ensures that the indication error length of thickness of the strip steel head part is controlled.