The invention discloses a rotational joint mechanical arm gravity compensation mechanism, which comprises a rack, a non-circular gear planetary gear train and a plane volute spiral spring. The rack comprises a plane volute spiral spring outer end fixing rod; the non-circular gear planetary gear train comprises a tie bar, a planetary shaft, a rotary shaft, a fixed circular sun wheel, a planetary circular gear, a non-circular sun wheel and a planetary non-circular gear; the circular sun wheel is fixed onto the rack through a flange; the tie bar is connected with the planetary shaft in a sleevingway; two ends of the planetary shaft are connected with the planetary circular gear and the planetary non-circular gear; the planetary circular gear is meshed with the fixed circular sun wheel; the planetary non-circular gear is meshed with the rotary non-circular sun wheel; the rotary shaft rotates along with the non-circular sun wheel; the inner end of the plane volute spiral spring is connected with the rotary shaft through a pin; the outer end of the plane volute spiral spring is connected with a fixing rod of the rack; the tie bar is an input component; and the non-circular gear planetary gear train enables the rotary shaft to output a certain amount of turning angles, and the plane volute spiral spring is further torqued, so that an anti-balance moment is produced. The rotational joint mechanical arm gravity compensation mechanism provided by the invention is simple in structure, stable and reliable to move, high in gravity compensation precision, and applicable to a continuousrotating mechanism.