In this paper, a tunable extrinsic chiral metasurface with giant circular dichroism in the optical frequency band is proposed. The unit cell of the metasurface consists of two symmetrical square silver split rings and a GST film sandwiched between them. Compared with the works reported in the existing literature, the CD of the metasurface is larger and the tuning range is wider. In the frequency range of 50 THz~300 THz, the CD of this extrinsic chiral metasurface is up to 0.85 when the GST is amorphous. Due to the large loss of the crystalline GST, the extrinsic chiral response is weakened and the maximum CD is 0.52 in the crystalline state. The GST switches between two phase states (amorphous-crystalline) and enables the frequency tuning range to reach about 70 THz. Further studies have shown that the CD can be tuned by changing the incident angle and the geometric parameters of the GST layer. When θ = 0° and φ= 0°, the CD is zero in all frequency bands, which means when the wave is incident normally, there is no intrinsic chiral characteristics when the wave is incident normally. The electric field distributions at the resonance point in different phase states are also investigated. This work provides a way to realize devices such as efficient polarization modulation devices, circular polarizers, and polarization filters in the optical frequency band.We propose a metasurface with tunable circular dichroism extrinsic chiral based on the phase change material Ge₂Sb₂Te₅ (GST). The metasurface is composed of two symmetrical square silver split ring resonators and a GST intermediate layer arranged periodically. Combined with the oblique incident light, this metasurface is capable of achieving electromagnetic properties similar to that of the chiral structure. Numerical simulation results show that in the frequency range of 50 THz~300 THz, the maximum circular dichroism (CD) of the metasurface is 0.85 in the amorphous state of GST and 0.52 in the crystalline state of GST. When the GST switches between two states (amorphous-crystalline), a tunable frequency of about 70 THz is achieved. Compared with the reported works, the CD of this metasurface is larger and the tuning range is wider. By studying the electric field distribution, the origin of the circular dichroism is explained; and the effects of incidence angles and structural parameters on the CD of this metasurface are investigated. This research will have potential applications in efficient polarization modulation devices, circular polarizers, and polarization filters in the optical frequency band.