Corn starch was used as raw material to produce film-forming agent for glass fiber sizing in this study. The starch was modified with isoamylase firstly, and then reacted with epoxypropane. The optimum conditions were obtained via orthogonal text, emphasizing the effects of the dosage of propylene oxide, sodium sulfate and sodium hydroxide, the temperature and the time of etherification. The optimum reaction conditions were obtained: etherification time etherification time 12 h，etherification temperature 50℃, propylene oxide 4g, sodium sulfate 0.24g, sodium hydroxide 4.8g. The results show the scenario could ensure the glass fibre forming process smoothly.