A velocity measuring device includes an event sensor, a ranging sensor and a controller. The event sensor could detect a first image frame of an object along a plane at a first time point and detect a second image frame of the object at a second time point. The ranging sensor could detect a first depth of the object along a depth direction at the first time point, wherein the depth direction is substantially perpendicular to the plane and detect a second depth of the object along the depth direction at the second time point. The controller could obtain first-dimensional velocity and a second-dimensional velocity along the plane according to the first image frame, the second image frame, the first depth and the second depth, and obtain a third-dimensional velocity along the depth direction according to the first depth or the second depth.