A biochip detecting device for detecting a biochip is provided. The biochip receives an incident light to produce an excitation light. Both the incident light and the excitation light include a specific wavelength. The biochip detecting device includes a light source producing the incident light, an optical attenuator, a filter, a sensor, and a control module electrically connected to the light source and the sensor. Light with the specific wavelength passes through the filter. The optical attenuator disposed between the light source and the filter attenuates an intensity of the incident light, and replaces the biochip. The sensor detects an intensity of the light with the specific wavelength attenuated by the optical attenuator, and generates an intensity signal. The control module adjusts the intensity of the incident light according to whether the intensity signal is complied with a predetermined requirement. A detection method for the light source is provided.