The present invention provides a manually operated projection screen having a decelerating and locking mechanism which intergrates a decelerating mechanism and a locking mechanism as a whole. The decelerating and locking mechanism for a projection screen includes a braking and locking device having a centrifugal brake shoe, a brake drum interacting with the centrifugal brake shoe to perform decelerating function, and a ratchet interacting with the centrifugal brake shoe to perform locking function. The decelerating and locking mechanism includes at least three sets of planetary gear, the transmission ratio of the planetary gear is 1:32 or above. The decelerating and locking mechanism and the projection screen using the same is compact and easy to assemble. The projection screen can be freely positioned or locked at any moment and is free of crease.