A photo-detecting apparatus includes a first photodetector, a second photodetector, a first modulation signal generating circuit and a second modulation signal generating circuit. The first is configured to generate at least a first detecting signal according a first modulation signal. The second photodetector is configured to generate a second detecting signal according a second modulation signal. The first modulation signal generating circuit is coupled to the first photodetector and operated between a first voltage and a second voltage, and is configured to generate the first modulation signal. The second modulation signal generating circuit is coupled to the second photodetector and operated between the second voltage and a third voltage. The second modulation signal generating circuit is configured to generate a second modulation signal. The value of the second voltage is between the first voltage and the third voltage.