A method comprising: S1. dividing attack and defense processes associated with a computer network system into first one or more single-node attacks and second one or more link attacks based on attack granularity; S2. determining one or more single-node attack parameters associated with the computer network system based on configuration of the computer network system; S3. calculating (1) success probability and (2) time length of a single node attack by applying GSPN theory to mathematically analyze the determined one or more single-node attack parameters; S4. based on the success probability of the attacking single node as a parameter, using the Markov chain and the Martingale theory to calculate an average number of steps required for a successful link attack; and S5. calculating a total theoretical average attack time based on the average number of steps required for a successful link attack and the time length of a single node attack.