The invention discloses a power supply system of a multi-split air conditioner, a power supply method and an air conditioner. The power supply system comprises a host machine and at least one slave machine, and is characterized in that the host machine is connected with a power grid, comprises a rectifying circuit and a main inverting circuit, and is used for rectifying alternating current provided by the power grid into direct current through the rectifying circuit and inverting first sub direct current obtained through shunting into first alternating current provided for a load connected with the host machine to use through the main inverting circuit; and any one of the slave machines comprises a slave inverting circuit, and any one of the slave inverting circuits is electrically connected with the rectifying circuit of the host machine and is used for receiving second sub direct current obtained through shunting of the direct current and inverting the second sub direct current into second alternating current provided for a load connected with the slave machine to use. According to the invention, a technical problem of a multi-split air conditioner power supply system in the prior art that the multi-split air conditioner power supply system is caused to be complex in circuit structure because a scheme that the host machine and the slave machine obtain electric energy from the power grid simultaneously require both the host machine and the slave machine to deploy an independent power supply circuit is solved.