A fan control device for controlling a fan comprising: a communication-processing module, receiving a fan-controlling instruction generated by an external system to generate a control signal, generating a detection signal according to a detection request of the external system, and providing state information for the external system; a fan-controlling module, controlling a speed of the fan according to the control signal, and transmitting a control result to the communication-processing module; and a detection module, detecting an ambient state and a fan state of the fan according to the detection signal, and generating the state information according to the ambient information and the fan state, wherein the fan-controlling module, and the detection module are physically separated from one another, and there is no data transmission between the fan-controlling module and the detection module to achieve a purpose of cost reduction, wherein the detection module comprises: a temperature-detecting unit, detecting temperature information according to the detection signal, wherein the temperature information comprises ambient-temperature information and fan-temperature information; a speed-detecting unit, detecting speed information of the fan according to the detection signal; a current-detecting unit, detecting current information of the fan according to the detection signal; and a voltage-detecting unit, detecting voltage information of the fan according to the detection signal, wherein each of the temperature-detecting unit, the speed-detecting unit, the current-detecting unit, and the voltage-detecting unit comprises a sensor, and the communication-processing module, the temperature-detecting unit, the speed-detecting unit, the current-detecting unit, and the voltage-detecting unit are physically separated from one another. a communication-processing module, receiving a fan-controlling instruction generated by an external system to generate a control signal, generating a detection signal according to a detection request of the external system, and providing state information for the external system; generated by an external system to generate a control signal, generating a detection signal according to a detection request of the external system, and providing state information for the external system; a fan-controlling module, controlling a speed of the fan according to the control signal, and transmitting a control result to the communication-processing module; and a detection module, detecting an ambient state and a fan state of the fan according to the detection signal, and generating the state information according to the ambient information and the fan state, wherein the fan-controlling module, and the detection module are physically separated from one another, and there is no data transmission between the fan-controlling module and the detection module to achieve a purpose of cost reduction, wherein the detection module comprises: a temperature-detecting unit, detecting temperature information according to the detection signal, wherein the temperature information comprises ambient-temperature information and fan-temperature information; a speed-detecting unit, detecting speed information of the fan according to the detection signal; a current-detecting unit, detecting current information of the fan according to the detection signal; and a voltage-detecting unit, detecting voltage information of the fan according to the detection signal, wherein each of the temperature-detecting unit, the speed-detecting unit, the current-detecting unit, and the voltage-detecting unit comprises a sensor, and the communication-processing module, the temperature-detecting unit, the speed-detecting unit, the current-detecting unit, and the voltage-detecting unit are physically separated from one another. a temperature-detecting unit, detecting temperature information according to the detection signal, wherein the temperature information comprises ambient-temperature information and fan-temperature information; a speed-detecting unit, detecting speed information of the fan according to the detection signal; a current-detecting unit, detecting current information of the fan according to the detection signal; and a voltage-detecting unit, detecting voltage information of the fan according to the detection signal, wherein each of the temperature-detecting unit, the speed-detecting unit, the current-detecting unit, and the voltage-detecting unit comprises a sensor, and the communication-processing module, the temperature-detecting unit, the speed-detecting unit, the current-detecting unit, and the voltage-detecting unit are physically separated from one another.