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Taking threshold energy epsivT into accounting for electron multiplying, the formula of silicon critical electric field ES,C is given as a function of silicon film thickness ts from an effective ionization rate alphaeff. ES,C is increasing with the decreasing of ts especially at thinner ts. 2-D simulative and some experimental results as well as the comparing with several other familiar expressions of electric field proved the proposed ES,C is valid for both thick and thin silicon film. Thin silicon film with high ES,C can be used to enhance dielectric field EI and increase vertical breakdown voltage VB,V of SOI high voltage devices. A high voltage SOI device with 80 nm silicon layer and 0.4mum dielectric layer is designed. A high ES,C of 134V/mum is obtained, which makes EI and VB,V reach to 416 V/mum and 178 V, respectively.