A speed detection method is implemented in a speed detector comprising a range finder. First, a specific number of distance measurement samples of an object are acquired during a period of time utilizing the range finder. The distance measurement samples are converted to M versions of the distance measurement samples respectively based on M velocities, wherein M is a positive integer. Each version of the distance measurement samples comprises reflected distance measurement laser pulses. Each reflected pulse corresponds to a measured distance and a received pulse order. A version of the distance measurement samples is determined, which comprises a distance range to which the highest number of reflected pulses corresponds. A speed of the object is calculated based on an assumed velocity to which the determined version corresponds.