The risk of people under building fire is expressed by genetic algorithms (GAs) reliability index β. Elementary program of genetic algorithm is basically discussed. This method is convenient to calculate evacuation reliability when performance function of evacuation is complex and difficult to get differential coefficient. Being compared with the result based on Monte-Carlo, the result of case analysis shows its efficiency, and genetic algorithm can be used to calculate the evacuation reliability with high precision. GAs can give a good new way to study the reliability of evacuation in case of building fire.