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Covert communication promises a secure transmission mechanism for Backscatter Communication (BC) by hiding the transmission behavior of backscatter devices from potential eavesdroppers. This letter investigates the upper limit of the covert rate in a scenario where a multi-antenna eavesdropper is present in BC setup, considering the inherent structural reflection of a backscatter tag. Specifically, the transceiver can employ a Gaussian signal to illuminate the tag, with special considering two distinct signals: a modulated signal and an additional reflected signal from the tag’s antenna structure. This letter considers a powerful eavesdropper who is equipped with a massive antenna to enhance its ability to detect the tag’s transmission activities. The results reveal the presence of the square root law ( $\Theta (\sqrt {n})$ covert bits in n channel uses) across various scenarios. Theoretical analysis demonstrates the inhibitory effects of both the tag’s antenna structural reflection and the variance of the transceiver’s Gaussian signal on the eavesdropper’s detection capabilities. Furthermore, numerical results illustrate the impact of different parameters on the covertness of the system.