The basic procedure for the chemical silvering for optical surfaces is introduced, the principle and technology of the sensitization is reported, and the theoretical analysis and experimental validation of water spraying is given. An experiment was performed using a secondary mirror with 365mm in diameter. By spraying water for 35 seconds, the Cl-in sensitizer can be cleaned thoroughly, and the Sn(OH)2 gelinite is left on the optical surface uniformly. The silver coating is very good. The silver coating has the 92% reflectivity, 60nm thickness, PV=λ/45 and RMS=λ/250 in surface change, which meets the coating requirements for the secondary mirror.