To prevent ergonomic injuries, proper assessment of ergonomic risk is a key to identify risk factors and modify work practice in a timely manner, and a fuzzy logical approach to posture-based ergonomic evaluation tools is proposed. Rapid Upper Limb Assessment (RULA) is selected as a case study to describe the fuzzy logic modelling of RULA scoring systems. The trapezoidal functions are used to represent the angle ranges of the inputs as well as the force and muscle use imposed on the worker, and the triangular functions are used for intermediate variables which represent the RULA intermediate scores and the final RULA total score. The results of validation reveal that the proposed system produces high correlation coefficients with RULA (0.937, p < 0.01) and OWAS (0.725, p < 0.01). By taking the control console of a enterprise as a case, two participants completed six separate operation tasks, the model was used to analyze 12 tasks. The result indicated that the model had high reliability and could help to minimize human errors in observation for reliable on-site ergonomic assessment.