Abstract: This paper presented a sensitivity analysis method for tracing the volumetric errors of a machine tool that made the greatest impact on machining errors based on Monte Carlo simulation theory. A machining error model of the five-axis gantry machine was established by means of a multi-body system theory and a mathematical model of S-shaped test piece was also established. The mapping relationship between the cutting points and tool paths was deduced. The mathematical model of cutting surface was then found. According to the principle of Monte Carlo simulation, a global sensitivity analysis was performed on the machining error model. Then, the geometric error parameters that make a great impact on machining errors were obtained. By sorting the sensitivity analysis results, five geometric errors δ_z(x), δ_z(y), δ_z(z), δ_z(c), and δ_z(b) made the greatest influence on the machining error model of z-axis. Experimental results show that when working together with the machine, the five key error parameters make an average influence on z-axis machining error up to 88.2%. Thus, the five error parameters make a greatest influence on z-axis machining errors. This method can provide theoretical guidance for improving the machining precision and design precision of the five-axis machine tool.