We report ultrafast third-order nonlinear optical (NLO) properties of several chalcogenide glasses GeSx (x = 1.8,2.0, 2.5) measured by femtosecond time-resolved optical Kerr gate technique at 820nm. The third-order nonlinear susceptibility of GeS1.8 glass is determined to be as large as 1.41 × 10-12 esu, which is the maximum value of the third order nonlinear susceptibility X(3) for the three compositions investigated. The symmetric Gauss profiles of optical Kerr signals reveal the nature of ultrafast nonlinear response of these samples, which are originated from the ultrafast polarization of the electron clouds. By detailed microstructural analysis of these glasses based on the chain-crossing model (CCM) and the random-covalent-network model (RCNM), it can be concluded that X(3) value of GeSx glasses can be enhanced greatly by S-S covalent bonds or S3Ge-GeS3 ethane-like units.