The paper investigates the contest behavior in outsourcing service competition by applying contest theory, and constructs the outsourcing service competition model based on contest model. On the basis of analyzing equilibrium of outsourcing service competition, it studies the equilibrium of outsourcing service competition with sequential decision order, and explores the effect of sequential decision order on competition equilibrium. The conclusion shows that the sequential decision order not only makes the equilibrium service level of the suppliers different, but also influences both total equilibrium service level of the supplier and the average equilibrium service level acquired by the purchaser.