Solar Irradiance Monitor-II (SIM-II) onboard FengYun-3C (FY-3C), the third satellite of the second generation of Chinese meteorological polar orbit series, is a successive instrument from FY-3A/B SIMs with improved capabilities of solar tracking and temperature controlling. SIM-II is aimed to observe incoming solar energy over 0.2-50 mm band at top of atmosphere and convert it to total solar irradiance (TSI) at the average sun-earth distance. The instrument has a measuring requirement of 0.1% absolute accuracy and long-term relative precision of 0.02% during its 4-year design lifetime. The TSI observed by SIM-II is analyzed and validated by comparing with SORCE/TIM and the composite time series from Royal Meteorological Institute of Belgium (RMIB) daily mean products. It shows that there is a good consistency on variation trend in TSI among SIM-II, TIM and RMIB, from March 5, 2014 to September 22, 2014. The quantitative comparison shows SIM-II daily TSI product has an 87*10~(-6) rms relative error compared with SORCE/TIM data and 140*10~(-6) rms relative error with RMIB composite data. The sunspot data including sunspot number and area is also analyzed with SIM-II TSI product. The sunspot number is negative correlation with SIM-II TSI daily product at solar activities. The TSI variation at the strongest solar activity during the October 2014 shows a good correlation with the sunspot area data. When sunspot area is increased by 2470, the variation in TSI from SIM-II is about -2.94 and -2.98 W/m~2 for TIM, which is very close to each other. It can be concluded that the SIM-II TSI observations can quantitatively reflect the variations of the solar emission during solar activities. These studies show that FY-3C TSI product had achieved the same accuracy as those of SORCE/TIM and RMIB data, and can be used in the researches of solar activities and climate changes.