Mobile cloud is a machine-to-machine service model, where a mobile device can use the cloud for searching, data mining, and multimedia processing. To protect the processed data, security services, i.e., encryption, decryption, authentications, etc., are performed in the cloud. In general, we can classify cloud security services in two categories: Critical Security (CS) service and Normal Security (NS) service. CS service provides strong security protection such as using longer key size, strict security access policies, isolations for protecting data, and so on. The CS service usually occupies more cloud computing resources, however it generates more rewards to the cloud provider since the CS service users need to pay more for using the CS service. With the increase of the number of CS and NS service users, it is important to allocate the cloud resource to maximize the system rewards with the considerations of the cloud resource consumption and incomes generated from cloud users. To address this issue, we propose a Security Service Admission Model (SSAM) based on Semi-Markov Decision Process to model the system reward for the cloud provider. We, first, define system states by a tuple represented by the numbers of cloud users and their associated security service categories, and current event type (i.e., arrival or departure).We then derive the system steady-state probability and service request blocking probability by using the proposed SSAM. Numerical results show that the obtained theoretic probabilities are consistent with our simulation results.