The advantage of optomechanical systems is that subsystems of different frequencies can be entangled by interaction with mechanical oscillators. In this paper, the entanglement of two longitudinal modes entangled by light-force coupling under special conditions is studied. The influence of filter function and input-output relation is considered in covariance matrix. The results show that there is still steady-state entanglement between the two optical modes of the output, and larger entanglement can be obtained by optimizing the correlation filtering parameters. This is of great reference value for the preparation and output of entangled light fields in practical experiments.