A method for controlling a rotational speed of a fan, comprising: an initialization step including using a control unit to start a motor of the fan according to an initial duty cycle value; characterised by a first constant speed step including using the control unit to compare a motor rotational speed with a first target rotational speed, wherein the duty cycle value outputted by the control unit is changed to make the motor rotational speed equal to the first target rotational speed when the motor rotational speed is different from the first target rotational speed, wherein when the motor rotational speed is equal to the first target rotational speed, it is determined whether the duty cycle value and a first target duty cycle value satisfy a first relational expression, if not, the first constant speed step is repeated; and a second constant speed step: wherein when the duty cycle value and the first target duty cycle value satisfy the first relational expression in the first constant speed step, the control unit compares the motor rotational speed with a second target rotational speed, wherein the duty cycle value outputted by the control unit is changed to make the motor rotational speed equal to the second target rotational speed when the motor rotational speed is different from the second target rotational speed, wherein when the motor rotational speed is equal to the second target rotational speed, it is determined whether the duty cycle value and a second target duty cycle value satisfy a second relational expression, if not, the second constant speed step is repeated, and if so, the first constant speed step is performed. an initialization step including using a control unit to start a motor of the fan according to an initial duty cycle value; characterised by a first constant speed step including using the control unit to compare a motor rotational speed with a first target rotational speed, wherein the duty cycle value outputted by the control unit is changed to make the motor rotational speed equal to the first target rotational speed when the motor rotational speed is different from the first target rotational speed, wherein when the motor rotational speed is equal to the first target rotational speed, it is determined whether the duty cycle value and a first target duty cycle value satisfy a first relational expression, if not, the first constant speed step is repeated; and a second constant speed step: wherein when the duty cycle value and the first target duty cycle value satisfy the first relational expression in the first constant speed step, the control unit compares the motor rotational speed with a second target rotational speed, wherein the duty cycle value outputted by the control unit is changed to make the motor rotational speed equal to the second target rotational speed when the motor rotational speed is different from the second target rotational speed, wherein when the motor rotational speed is equal to the second target rotational speed, it is determined whether the duty cycle value and a second target duty cycle value satisfy a second relational expression, if not, the second constant speed step is repeated, and if so, the first constant speed step is performed.