In stellarators, error fields arise from the inevitable deviations in the fabrication and assembly of complex coil systems. The magnetic configurations of stellarators are predominantly generated by external coils and are highly sensitive to these error fields. Therefore, assessing the impact of coil deformations on stellarator magnetic topology is important. The purpose of this study is to explore the influence of error fields, caused by modular coil (MC) perturbations, on the magnetic topology of the Chinese First Quasi-axisymmetric Stellarator (CFQS). In this work, by changing the Fourier coefficients that represent the current-carrying surface (CCS) and the coil, two types of deformation coils, i.e. “in-surface” and “out-of-surface” disturbance on each MC can be obtained. Subsequently, three kinds of magnetic islands (ι = 2/4, 2/5 and 2/6) are used to identify coil deviations that have a significant influence on the CFQS magnetic configuration. Several important results are obtained as follows. i) The same deformation of a coil gives rise to various resonant error fields with different amplitudes. ii) The sensitivity of a resonant error field to the deformation of each coil is different. The in-surface disturbance of the most complex coil may not have a significant influence on the magnetic topology structure. iii) The sensitivity of the resonant error field to out-of-surface disturbance in the coil is higher than that to in-surface disturbance.