The emerging compressed sensing (CS) technique enables new reduced-complexity designs of sensor nodes and helps to save overall transmission power in wireless sensor network. Because of the linearity of its encoding process, CS is vulnerable to Ciphertext-Only Attack (COA) and Known-Plaintext Attack (KPA). The prior works use multiple sensing matrices as the shared secret key, however, the complexity overhead of front-end sensor and synchronization issue arising from multiple keys should be well considered. In this paper, by leveraging the characteristic of CS that is sensitive to destroyed sparsity, a low-dimension watermark is randomly chosen and embedded in measurement in front-end part. Then, in back-end solver, the proposed decrypting basis can decipher the encrypted signals without synchronization. Simulation results show that the proposed scheme achieves effective protection against COA and KPA with only 5% storage overhead. It furtherly eases the encryption complexity of front-end sensor by 98.8% under our experiments.