A power on control device and method are used at an adaptor providing a constant voltage. The power on control device includes a first voltage converting circuit, a second voltage converting circuit, a third voltage converting circuit, and a comparison module. The first voltage converting circuit receives the constant voltage and generates a reference voltage. The second voltage converting circuit receives the constant voltage and generates operating voltages. The third voltage converting circuit receives the operating voltages and generates preparatory voltages corresponding to the operating voltages according to the operating voltages. The comparison module outputs a power good signal when all the preparatory voltages are larger than the reference voltage.