The array piezocrystals sensor and the conditioning circuit of array signal were designed. The software and hardware of monitoring system were completed based on the single chip microcomputer, and the instrument of monitoring grain cleaning loss was realized. Experimental data showed that the response time of the intelligent monitor for grain was less than 1s, and the measurement errors were less than 5%. The system effectively cancelled all kinds of interference during harvesting, and could monitor grain cleaning loss on line with high accuracy.