A power supply of a transformer outputs alternating current (AC) and direct current (DC) and contains: a housing and a lid for covering a first end of the housing. An input interface is formed on a second end of the housing, and the lid includes a DC output interface and an AC output interface. A light string contains a power supply of a transformer and a light emitting unit, the power supply outputs alternating currents (AC) and direct currents (DC), and the power supply contains: a housing and a lid for covering a first end of the housing. An input interface is formed on a second end of the housing, the lid includes a DC output interface and an AC output interface, and the DC output interface couples with a DC output connector, and the DC output connector connects with plural bulbs of the light string.