The total sky imager(TSI-440) is designed to automatically monitor cloud conditions at daytime with higher spatial resolutions and is able to provide more accurate cloud cover data.The paper introduced the fundamental mechanism and data format of the TSI in detail and then analyzed the imaging characteristics under different weather conditions and the cloud cover computational errors based on the routine surface observation data from TSI-440 at Wuxi from May to October 2008 by using statistical method.The results show that the imaging characteristic is closely related with the visibility and the red blue ratio increases as the visibility reduces.After analysis of the images,it is also found that the TSI-440 makes certain errors in computing cloud cover when processing cloudy images under overcast or cloudy conditions.In view of this situation,the paper sets new thresholds(sunny points at 0.62;cloudy points at 0.66) to simulate the sky images and count the cloud points.The recalculated results are much more reliable than the raw results from the TSI.Setting new thresholds could reduce the cloud cover errors effectively,especially on cloudy days.