The invention discloses a construction method and structure of an underwater reinforced concrete combined suspension-box cofferdam free of bottom-sealing concrete. The construction method comprises the following steps: after pile foundation construction is finished, manufacturing a suspension box; mounting carrying pole beams and continuous jacks on pile casings as lowering devices of the suspension box; slowly lowering the suspension box in place with the jacks; and pouring underwater undispersed concrete between a bottom board and annular stepped grooves of the pile casings. According to the technological method, the suspension box stops water by virtue of the reinforced concrete bottom board and the underwater undispersed concrete, which can thoroughly break through the conventional construction method in which bottom sealing is indispensable in construction of an underwater bridge/wharf pile cap suspension box cofferdam, saves a great amount of concrete and steel, reduces the construction cost, simplifies construction procedures and shortens the construction period; and the method is simple in process and convenient to operate, has a good water-stopping effect and a wide application range, has low requirements on ambient environment, corollary equipment and operation level of workers, and has high popularization and application values.