Abstract: One way to evaluate high power microwave's effect to computers in the building is proposed. Finite-difference Time-domain method is used to compute the electromagnetic field in the building and we can get the greatest field of every point in the building. The volume ratio which is greater than the effect threshold equals the computer's effect probability. An experiment was carried out to test the method. The computer's effect ratio difference between the experiment and the computation is less than 20%. By changing the incident field intensity, the relation between the computer effect ratio and the incident was analyzed. The experimental results agree well with theoretical calculation results.