The invention provides a photovoltaic direct current system and a low voltage crossing control method thereof, when the DC bus voltage drops, At first, that DC bus voltage and the pas power of the DC/DC convert are obtained, Then, according to the crossover power, the corresponding voltage value when the output power of the photovoltaic module is the crossover power is searched on the maximum power tracking curve of the photovoltaic module, the voltage value is taken as the crossover voltage of the corresponding photovoltaic module, and finally, the output crossover power and the crossover voltage of the photovoltaic module are controlled. According to the technical proposal provided by the invention, when the DC bus voltage drops, without changing the original photovoltaic DC system, Theoutput of the photovoltaic module can be controlled according to the power of the DC/DC grid-connected side, so that the current of the DC/DC converter grid-connected side can be kept unchanged, and the DC/DC converter will not be impacted by the DC bus bar, thus solving the problem that the photovoltaic DC/DC converter is off-grid.