The conversion from horizontal adit driving to main tunnel driving of Metro works has great construction difficulties and complex construction procedures. It is proposed, on basis of construction scheme comparison, that full-envelope construction method should be adopted for the conversion from horizontal adit driving to main tunnel driving. The works has already been successfully completed. The works practice demonstrates that full-envelope construction method can provide safe and reliable construction and significant technical, economical and social benefits. The paper can provide reference for similar works in the future.