Abstract: This paper introduces the related knowledge of H-4 subsea connector in detail. Through the force analysis of its main functional component 'locking ring', the reason that the connector can seal the higher wellbore pressure with lower control pressure is revealed. Through the force analysis of another functional component 'latch segment', the essential reason for the unlocking failure of the connector is revealed. Combined with other theories and calculations, the phenomena in the two typical cases are explained. Finally, the solutions, treatment suggestions and preventive measures for the unlocking failure of H-4 connector are put forward for the reference of relevant practitioners.