A lens assembly fixing device of scanner, wherein the scanner includes a body, a top cover, an upper surface plate and a scanning lens assembly. The scanning lens assembly is set inside the scanner and can move to and fro horiziontally and longitudinally therein. At a desired location of a side of the lens assembly is mounted a positioning board having a diamond-shaped opening thereon, and a socket and a spring bolt are installed on the upper surface plate and the top cover respectively, corresponding to the top of the diamond-shaped opening when the scanner is in standby mode. It prevents the lens assembly from being damaged by collision generated during its sliding randomly at removal, as a result from that the spring bolt inserts into the diamond-shaped opening on the positioning board through the socket on the upper surface plate when the scanner stops operation. The locking of the spring bolt will automatically be released, as soon as the top cover is opened in each use or operation.