Underwater simultaneous casing hole forming method

The invention provides an underwater simultaneous casing hole forming method which comprises the following steps of: performing drilling in an underwater filling body by using an off-balance bit of ahigh wind pressure down-the-hole drill and a steel casing, continuing drilling to a designed elevation by changing the off-balance bit of the high wind pressure down-the-hole drill to a conventional bit after the steel casing enters bedrock, taking out a drilling rod and the conventional bit, mounting a PVC (polyvinyl chloride) casing in the steel casing after finishing the drilling, and pulling out the steel casing. Simultaneous casing hole forming for underwater drilling and blasting operation is achieved by the above steps. The method can achieve the efficient and accurate underwater drilling in underwater drilling and blasting construction; the underwater drilling efficiency of single CM351 can reach 150-200 per shift; and an underwater drilling period in large-scale underwater blasting operation is greatly sh