The warship equipment’s spare demand is difficult when it executes the mission on the distant seas. The models and computational problem of operational readiness are studied based on limited spare number. Firstly, the model of operational readiness for one unit is set up based on limited spare. The method of  approximate calculation is deduced for engineering application expediently. Secondly, the models of operational readiness for system are set up based on limited spare. The algorithm of operational readiness for series connection, parallel connection and voting connection is researched specially based on spare sharing, and then it is given that the algorithm path of operational readiness for system. Finally, through simulation the steady-state availability is inapplicable for warship equipment when it sails on the distant seas. The calculation methods of operational readiness for unit and system are of high precision. The study can be used for operational readiness assessment.