In this paper, a design-for testability (DFT) technique is presented to diagnose switched-capacitor (SC) circuits. In order to avoid the effect that pure DC signal cannot pass through un-switched capacitors, we use semi-DC signal to diagnose SC circuits. Furthermore, we propose a controllable opamp that it can be controlled to normal mode or test mode. In normal mode, it passes signal normally; in test mode, it provides a semi-DC test signal (VDD or VSS) and blocks the signals from the stage before controlled stage. In our diagnosis method, we consider faults both in capacitors and in opamps. Experiments have carried out to verify the practicality of this technique.