A power supply device for a solid state drive (SSD) inserted into a slot of a computer includes a number of voltage input contacts, a diode, a first voltage regulating module and a second voltage regulating module connected in series. The slot includes a number of voltage output contacts and a number of signal transmitting contacts. The voltage input contacts are connected to the voltage output contacts and selectively connected to the signal transmitting contacts. The voltage input contacts obtains an initial voltage from the slot from the voltage output contacts or the signal transmitting contacts. The first voltage regulating module boosts the initial voltage. The second voltage regulating module regulates the boosted voltage to a preset voltage. The diode prevents a voltage outputting from the voltage input contacts to the signal transmitting contacts.