Quantum key distribution (QKD) technology provides proved unconditional security in pointto-point key transmissions based on quantum mechanics. However, several limitations, most prominently range, make it complex to construct large QKD network. The trusted relay QKD network is the most practical scene, and stochastic routing scheme was proposed to solve the fatal problem that some relay nodes may be controlled by adversary. But most results in stochastic routing scheme hinge on an adversary model without consideration of attacker's prior-knowledge and attack strategy. To reveal the security problem, we propose a new adversary model and apply it in grid-shaped partially-trusted relay QKD network. Then we analyze the best attack strategy and the impact on security under various situations respectively. Analysis and simulation results show that in some cases the security of stochastic routing scheme will be compromised, and a generalized upper bound for the overall safe probability is obtained. We also discuss how to alleviate the security problem, making the network more robust to attacks.