For a coal-based production system with entrained flow coal gasification, the fluctuations in syngas yield (SY) affect the stability of downstream sections directly. The focus of this study is to analyze how some important operating parameters affect these fluctuations in SY for a typical entrained flow coal gasification, Shell coal gasification process (SCGP). In this study, we carried out a set of numerical simulation experiments in Aspen Plus simulator for a demonstration SCGP gasifier. These experiments were arranged using a special design of experiment (DoE) method, split-plot design. Then we systematically analyzed the effects of pressure (P), oxygen to coal ratio (O/C), and steam to coal ratio (S/C) on the Range and standard deviation (StDev) of SY using some statistical techniques. The results show that split-plot design is an appropriate DoE method to investigate and analyze the effects of the operating parameters on the fluctuations in SY. For the demonstration SCGP process, the fluctuations in SY are mainly affected by P and O/C. Among them, P is the dominant factor and O/C is an important factor, while S/C influences the fluctuations in SY slightly. The lowest Range, and StDev of SY could be obtained only when P is at a higher level and O/C as well as S/C is at a lower level. This is the ideal condition for realizing a moderate robust syngas production.