A method for controlling a rotational speed of a fan (F), comprising: an initialization step (S0) including using a control unit to start a motor of the fan (F) according to an initial duty cycle value (D); characterised by a first constant speed step (S1) including using the control unit to compare a motor rotational speed (R) with a first target rotational speed ( R1 ), wherein the duty cycle value (D) outputted by the control unit is changed to make the motor rotational speed (R) equal to the first target rotational speed (R1) when the motor rotational speed (R) is different from the first target rotational speed (R1), wherein when the motor rotational speed (R) is equal to the first target rotational speed (R1), it is determined whether the duty cycle value (D) and a first target duty cycle value (D1) satisfy a first relational expression, if not, the first constant speed step (S1) is repeated; and a second constant speed step (S2): wherein when the duty cycle value (D) and the first target duty cycle value (D1) satisfy the first relational expression in the first constant speed step (S1), the control unit compares the motor rotational speed (R) with a second target rotational speed (R2), wherein the duty cycle value (D) outputted by the control unit is changed to make the motor rotational speed (R) equal to the second target rotational speed (R2) when the motor rotational speed (R) is different from the second target rotational speed (R2), wherein when the motor rotational speed (R) is equal to the second target rotational speed (R2), it is determined whether the duty cycle value (D) and a second target duty cycle value (D2) satisfy a second relational expression, if not, the second constant speed step (S2) is repeated, and if so, the first constant speed step (S1) is performed. an initialization step (S0) including using a control unit to start a motor of the fan (F) according to an initial duty cycle value (D); characterised by a first constant speed step (S1) including using the control unit to compare a motor rotational speed (R) with a first target rotational speed ( R1 ), wherein the duty cycle value (D) outputted by the control unit is changed to make the motor rotational speed (R) equal to the first target rotational speed (R1) when the motor rotational speed (R) is different from the first target rotational speed (R1), wherein when the motor rotational speed (R) is equal to the first target rotational speed (R1), it is determined whether the duty cycle value (D) and a first target duty cycle value (D1) satisfy a first relational expression, if not, the first constant speed step (S1) is repeated; and a second constant speed step (S2): wherein when the duty cycle value (D) and the first target duty cycle value (D1) satisfy the first relational expression in the first constant speed step (S1), the control unit compares the motor rotational speed (R) with a second target rotational speed (R2), wherein the duty cycle value (D) outputted by the control unit is changed to make the motor rotational speed (R) equal to the second target rotational speed (R2) when the motor rotational speed (R) is different from the second target rotational speed (R2), wherein when the motor rotational speed (R) is equal to the second target rotational speed (R2), it is determined whether the duty cycle value (D) and a second target duty cycle value (D2) satisfy a second relational expression, if not, the second constant speed step (S2) is repeated, and if so, the first constant speed step (S1) is performed. <EOS>