Compared with traditional auctions , online auctions have some unique characteristics , which in clude bidders ’ stochastic arrival , snipping effects , the existence of auction platform and the fee charged to sellers , as well as multi unit and multiple stages. Taking into account these unique characteristics , this paper explores some important questions of multi unit online auctions systematically. The result shows that goods with larger valuation disparity are more suitable to be on sale by auction but under the precondition that the network user basement should achieve a certain scale ;under a unified analysis framework , we solve the seller ’ s opti mal public reserve price and private reserve price respectively , and also verify that the seller with private re serve price can gain more profits than the seller with public reserve price ;considering the inconsistency be tween the auction platform and the seller ’ s objectives , we indicate that , to decrease the listing fee , increasing ommission ratio and shortening the auction duration are both helpful for us to design the incentive compatible optimal auction mechanism ;to auction more goods is not always optimal for the seller , we solve for the optimal number of goods to be sold in one auction under one stage and multiple stage auctions respectively.