To address the issues of insufficient privacy protection of quotations and collusion between malicious bidders and third parties in existing quantum sealed auction protocols, a quantum sealed-bid auction protocol based on semi-quantum secure direct communication is proposed. Firstly, the protocol adopts semi-quantum secure direct communication, in which the auctioneer only needs to measure and reflect particles during communication. Secondly, the bidder can achieve the privacy protection of quotations through order-preserving transformation. Thirdly, the auctioneer can achieve privacy comparison of the transformed bid information without the participation of a trusted third party. Theoretical analysis shows that the proposed protocol is highly secured even facing attacks such as intercept-resend attack, control-NOT (CNOT) attack, phase inversion, and collusion attack. In addition, compared with the existing similar quantum auction schemes,the communication efficiency of the new protocol is not affected by the number of bidders.