A handheld electronic device includes a first camera unit, a laser focusing module and a control module. The laser focusing module is configured for radiating a laser signal and receiving a feedback signal induced by a reflection of the laser signal when the first camera unit is activated. The control module is coupled with the camera unit and the laser focusing module. The control module is configured for monitoring a strength level of the feedback signal or a response time between the laser signal and the feedback signal, and selectively generating a command to trigger the first camera unit according to the strength level or the response time.