The invention discloses a modularization multi-level photovoltaic grid connected system and a control method thereof. A three-phase six-leg topology structure is employed each leg is formed by connecting a half-bridge module SM, N assembly modules PM and one inductor L in series. Each SM is composed of two IGBT tubes T1 and T1, two diodes D1 and D2, and one capacitor C. Each PM is formed by connecting one photovoltaic assembly PV at the two ends of the capacitor C on the basis of each SM. The control method comprises maximum power tracking control, redundancy module control, voltage stabilization control and grid connected control. According to the invention, reliable working of an inverter and maximum power tracking of the photovoltaic assemblies can be realized under the condition of local shadow, a modularization multi-level structure reduces capacitor stress and an inversion output harmonic wave content, a DC transformation circuit is unnecessary, through the control method, the maximum power tracking of each PV assembly is realized, compared to a conventional array global maximum power control method, the output power is improved, and hardware investment is reduced.