The photoisomerization and excited state intramolecular proton transfer (ESIPT) are the most common processes in molecular photoswitches. Their coexistence and competing relationship in molecules usually make the deactivation process of the molecules become complex. In the current work, we study the underlying deacti-vation mechanism of trans-form 2 '-hydroxychalcone (2 ' HC), which possesses both photoisomerization and ESIPT in the excited state, by combining high-level electronic structure calculations and on-the-fly surface hopping dynamics simulations. Four minimum energy conical intersections (MECIs) are found to be involved in the whole deactivation process. Following excitation to the S3 state of the most stable trans-enol tautomer, a strong coupling from the utut* onto the nut* state facilitates the ultrafast S3/S1 internal conversion process through an intricate conical intersection seam among S1, S2 and S3 state. The subsequent S1 to S0 state relaxation pathway is divided into two branches. The first process is initiated by twisting motion of C8C9 bridging bond, which decays directly to the ground state via enol MECI. The other one is triggered by an ultrafast ESIPT process firstly and followed by twisting motion of the C1C7 bond, leading the system funnel to the ground state by a keto type MECI, and finally generates the precursor of coloured flavanone. The proposed S3 ->(S2) S1 -> S0 two step decay pattern is consistent with previous experimental observations. At the molecular microscopic level, the C--O bond alternation motion plays a vital role in the whole relaxation process. Our results indicate that further practical application of 2 ' HC as molecular photoswitches can be achieved by modifying the molecular structure to enhance the ESIPT process.