Based on the optoelectronic transformation technology and the parallel-laser-scanning inspection principle, a transmission laser scanning inspection system for diameter measurement was designed. A parallel-flat-crystal was used to make up the transmission laser scanning transmitter so that highly paralleled scanning beam can be acquired in the absence of scanning objective lens. The front data processing module and the communication module were included in the measurement and control platform based on FPGA. The incident angle was determined by the time parameter which was measured by the peripheral timing circuit. The offset between transmission beam and incident beam was calculated through the incident angle. The diameter of work-piece was then derived by simple algebraic operation. The measuring principle and characteristics of the system were discussed in details. The experimental platform was established and results show that precision requirement on assembly of the transmitter is not so higher than other systems and measurement error can be easily limited in ±0.008 mm.