A computer input device includes a body and a trace-detecting module coupled to the body. The body has a micro control unit (MCU), and the trace-detecting module has at least a light pervious area, and a trace-detecting unit. The trace-detecting unit further has at least a light source and a sensor. The sensor senses a reflected light beam caused by movement of a user's digit movement on the light pervious area at a velocity which can be sensed by the sensor. If the velocity exceeds a threshold stored in the MCU, the MCU executes automatically scrolling.