The working principle of the Solar Irradiance Monitor (SIM) boarded on FY-3A meteorological satellite for monitoring solar irradiance was introduced and its system construction and working modes were described. The SIM was composed of three solar irradiance absolute radiometers (SIAR). The three SIARs were arranged with a certain angle to form three measuring channels to survey the solar irradiance simultaneously or independently and to compare, prove and correct the data obtained. Three kinds of working modes of the SIM were described, and then the tasks of SIM on-orbit was analyzed in detail. Furthermore, the software design and implementation of SIM were introduced and remote sensing data obtained on orbit with different measurement modes were given. The SIM on FY-3A has been worked normally on orbit from June 2008, and experimental results demonstrate that the measurement modes are reasonable, parameters for control and running are validity, and the task of monitoring solar irradiance are supported by the software functions designed. Moreover, the solar irradiance measured by the SIM is well coincident with that simultaneously measured by SORCE/TIM in 0.2%.