Astronomical site selection work is very hard. Unmanned technologies are important trends and solutions. We present a relatively easy method to plan a high reliability site selection which can extend the time from site deployment to returning for maintaining by unmanned confirming the site. First, we redefine the reliability of a site selection deployment with the parameter of the trusty time, which means when we must return, and which can be relatively easy for estimating. The redefinition makes the reliability parameter as a Bayesian probability, and can be obtained by estimating besides testing, which makes the evaluation of each device's reliability much easier. Then we use block diagram tools in the Matlab Simulink software to construct structure diagram, and to link each component with relations of parallel, serial, protection, and so on. This makes the whole reliability value can be calculated at the time when we design or plan a site selection. We applied this concept and method in an actual site selection in Lenghu, Qinghai Province, China. The survey practice reveals its effectiveness and simpleness.