With the wide use of NVM-based DNN accelerators for higher computing efficiency, the long data retention time essentially causes a high risk of unauthorized weight stealing by attackers. Weight encryption is an effective method, but existing ciphertext computing accelerators cannot achieve high encryption complexity and flexibility. This paper proposes WeightLock, a mixed-grained hardware-software co-design approach based on local decrypting units (LDUs). This work proposes a key-controlled cell-level hardware design for higher granularity and two weight selection schemes for higher flexibility. The simulation results show that the accuracy of VGG-8 and ResNet-18 in the Cifar-10 classification drops from 80% to only 10% even if 80% of keys are leaked. This shows >20% higher key leakage tolerance and >17x longer retraining latency protection, compared with the prior state-of-the-art hardware and software approaches, respectively. The area cost of the encryption function is negligible, with ~600x, 2.2x, and 2.4x reduction from the state-of-the-art cell-wise, column-wise, and 1T4R structures, respectively.