Precision instruments are tending towards intelligence and diversified functions, making the evaluation of measurement uncertainty in precision instruments more complex. Taking roundness measurement in CMM as an example, a task-oriented uncertainty evaluation method is used to quickly and reliably evaluate the uncertainty of CMM measurement tasks. Based on the actual situation of error propagation and combined with the Monte Carlo method of computer simulation, a method for algebraic sum synthesis of uncertainty under multiple states of error distribution is proposed. Aiming at the problem of overestimation in the initial evaluation of measurement uncertainty, an optimized evaluation scheme is proposed. Provide an example of uncertainty evaluation and optimization for roundness measurement tasks to verify the feasibility and effectiveness of the evaluation model and optimization methods. Systematically solving the uncertainty evaluation problem of CMM task oriented has important reference significance for solving the uncertainty evaluation of other high-precision instrument measurement results and improving the practical use value of the instrument.