The invention discloses a real-time detecting method of connecting line voltage zero-crossing in an SVC (Static Var Ccompensator) running process. (1) Zm(n-1) and Zc(n-1) are set as the (n-1)th actually measuring zero-crossing point and a predetermined zero-crossing point respectively, and Ik and Ik-1 are sampling points before and after the zero-crossing time; (2) Zm(n) and Zc(n) are the nth actually measuring zero-crossing point and the predetermined zero-crossing point respectively, and Ij and Ij+1 are sampling points before and after the zero-crossing time; (3) delta T(n+1) and delta T(n) are time differences between the (n-1)th and nth measuring zero-crossing point and the predetermined zero-crossing point, wherein the two zero-crossing points are superposed and the values of delta T(n+1) and delta T(n) are equal to zero in a stable state, and the two zero-crossing points are different in a transient/disturbance state, i.e. the values of delta T(n+1) and delta T(n) are not equal to zero; (4) a measurement zero-crossing point can be obtained by adopting a linear fitting method; (5) the relationship between the predetermined zero-crossing pint and the measurement zero-crossing point is Zc(n)=Zc(n-1)+T0/2+dataT(n-1)(kp+ki/s) (6), wherein T0 is sampling period, kP and ki are respectively coefficients of a proportional link and an integral link, s represents complex frequency of Laplace transform; and (6) three formulas (8), (9) and (10) are obtained after the formula (6) is discretized, wherein S is an error accumulator, Y is the correction quantity of the predetermined zero-crossing point Zc(n), and k is the ordinal number of the period.