Objective  Analyze astigmatism in 3-6 years old preschool children using vector analysis and to investigate the effect of cycloplegia on astigmatism. Methods  In this cross-sectional study， refraction and eye health were examined in 14 116 3-6 years old children （7 310 boys， 6 806 girls） in Yangpu District， Shanghai， China， from January 2012 to October 2012. Pre- and post-cycloplegic refractive measurements were taken with an autorefractor （Canon RK-F1， Japan） and axial length was measured with an IOLMaster （Carl Zeiss， Germany）. Of 13 080 eligible children， all the data from the right eyes were analyzed. Measurements of spherocylinder were transposed into M， J0 and J45 components by Fourier transformation. Data were analyzed using a paired t test. Results  Astigmatism in preschool children 3-6 years old was mainly with-the-rule （WTR）， especially in the high astigmatism group （HAG， ≥1.0 D）， followed by oblique axis astigmatism， and then against-the-rule （ATR） astigmatism. After cycloplegia， the prevalence of high astigmatism （≥1.0 D） decreased from 19.96% to 18.41%； the prevalence of normal astigmatism （0.25-0.75 D） increased from 28.59% to 33.72%. Meanwhile， the proportion of WTR astigmatism increased， and ATR astigmatism and the oblique axis astigmatism ratio declined slightly after cycloplegia. Total astigmatism declined only in the high WTR astigmatism group （-1.612±0.763 D vs. -1.457±0.645， t=-6.396， P<0.01）， mainly for J0 components. The J45 components remained the same before and after cycloplegia. Conclusion  Cycloplegia has a slight effect on astigmatism， especially in the high WTR astigmatism group. The astigmatism results for non-cycloplegia may be more important for preschool children when assessing and prescribing for astigmatism.