Photovoltaic (PV) units in DC microgrids (MGs) usually have two operating modes including constant voltage control (CVC) mode and maximum power point tracking (MPPT) mode. When the operating mode of PV units switches between CVC and MPPT, unexpected transient problems of output voltage and current may occur. Droop control applied in DC-MGs can achieve load current sharing at the time when more than two voltage-source units are connected in parallel. This paper puts forward a seamless switching control strategy of PV units in DC-MGs based on droop control. The proposed control strategy consists of voltage-current inner loops, droop control loop, and MPPT module. In this paper, conventional control strategy of PV units in DC-MGs is introduced firstly, and then the droop characteristics of voltage-shifting are analyzed. A basic DC-MG system made up of PV unit, DC load and AC grid interface inverter is set up to validate the proposed control strategy. The MPPT performance of proposed control strategy is tested. Experimental results show that compared with the conventional control strategy, better transient characteristic is achieved by using the proposed seamless switching control strategy.