The invention relates to a large-scale cast-in-situ reinforced concrete water inlet duct and scroll case construction method, the construction sequence is: bottom board of the water inlet duct, far end lower part outside the turning of the water inlet duct, far end upper part outside the turning of the water inlet duct, outer circumference of the turning of the water inlet duct, far end outside the scroll case, near end outside the scroll case, top outside the scroll case and the second pouring construction. The construction method has clear levels, reduces the concrete pouring amount at the key parts of the water inlet duct: the outer circumference of the turning of the water inlet duct and the near end outside the scroll case, by partitioning construction; ensures the concrete forming quality of the water inlet duct and the scroll case; and avoids large volume concrete cracks.