Based on the former "Local Nonlinear Feedback Loop (LNFL)" technique for stabilization of oversampling Sigma-Delta Modulator (SDM), an improved technique called "Sub-bit Nonlinear Feedback Loop (SNFL)" is proposed to further raise the stability. The SNFL technique not only avoids the instability limitation of high-order SDM without SNR degradation like the former LNFL technique, but also suppresses the output swing range of the critical integrator more effectively than the former LNFL technique. This is because the stabilization scheme of the SNFL technique can achieve to restrict the key signal within sub-bit swing range (- \Delta /4, \Delta/4) or less, while the LNFL technique only can restrict the key signal within integer-bit swing range (- \Delta /2, \Delta/2). The simulation verifies that both stability and dynamic range of SDM is improved by the proposed SNFL technique more than by the former LNFL technique.