In this study, a composite robust control approach with non-singular terminal sliding mode control and fixed-time disturbance observer (FxTDO) is proposed for a class of second-order non-linear systems in the presence of matched uncertain disturbances. A bi-limit-weighted homogeneous FxTDO is designed based on the efficient utilisation of power functions obtained by the differentiation error amplification strategy. The FxTDO enjoys a fast convergence rate and simple parameters tuning process. By introducing the disturbance estimation to directly compensate the total disturbance in the composite control law, only a particular small gain-based discontinuous control term is needed to guarantee the existence of sliding mode, and chattering phenomenon can be attenuated. The simulation results revealed the effectiveness of the proposed method.