Deflection problems are important causes of deterioration to the dimensional precision of machining micro thin-walled components. Under mesoscale, the deflection of the cutting tool and micro thin-walled workpiece model is established based on the Euler-Bernoulli cantilever beam theory. Then a novel average thickness prediction model of micro straight thin wall is set up considering the runout, cutting tool, and workpiece deflection. Afterwards, based on experimental data, the model is modified by introducing a scale factor K. This modified model has been calibrated and validated with Ti-6Al-4V titanium alloy, and it can provide very accurate prediction results for the micro straight thin wall at average thickness. In addition, the law of micro thin wall milling forces is also studied. In the end, the effect of different micro straight thin-walled stiffness on micro thin-walled dimensional accuracy is analyzed. The study found that for the thickness of micro straight thin walls, 100 μm is an inflection point to distinguish dimension precision between macroscale and mesoscale.