A sun tracking system was developed in order to improve the collection efficiency of solar air collector and receive more solar energy．Several main tracking modes were briefly introduced. The passive tracking mode was likely to be perplexed by weather condition and the active tracking mode could not avoid the cumulative error．A single axe automatic tracking mode which combined active tracking mode and passive tracking mode wais presented．The proposed combined mode overcame the shortcomings of the two modes．Experiments showed that this system works with high precision，good stability and little loss of solar energy．Automatic tracking system could make the collector outlet temperature increased an average of 15℃, so that the collector efficiency was improved.