The invention relates to a synchronous grid-connected method of a miniature inverter. The synchronous grid-connected method comprises the steps of detecting a three-phase voltage of a power grid side and a three-phase voltage of a miniature inverter side of a grid-connection point in real time; acquiring a symmetric power grid three-phase voltage and a symmetric miniature inverter three-phase voltage after the detected voltages are processed by an LC filter and a signal regulator; calculating a D-axis component and a Q-axis component of the symmetric three-phase voltage by DQ coordinate transformation calculation; calculating voltage difference, phase difference and frequency difference between a power grid and the miniature inverter; and forecasting voltage difference, phase difference and frequency difference of next moment by a Kalman filtering algorithm, judging whether the grid-connected requirement is met or not, if yes, performing grid-connected running by the miniature inverter, otherwise regulating and repeating the abovementioned steps by a diesel engine until the power grid is connected. By the synchronous grid-connected method, rapid and smooth grid connection can be achieved in grid connection of the miniature inverter, and harmonic interference can be suppressed very well.