In this letter, we propose a tag selection scheme to enhance the security transmission in a passive backscatter communication system with multiple tags and one eavesdropper, and analyze the secrecy outage probability (SOP). In particular, considering a practical non-linear energy harvesting model, we first determine the optimal dynamic reflection coefficient to maximize the power of the backscattered signal while satisfying the energy-causality constraint of tags, and then select the tag that can maximize the instantaneous secrecy capacity. The closed-form analytical expression of the SOP corresponding to the proposed tag selection scheme over the independent but not necessarily identically distributed Rayleigh fading channels is derived and the diversity gain is analyzed. Numerical simulations corroborate the theoretical analysis and exhibit the superior performance of the proposed scheme compared to other schemes.