The invention discloses a construction method for a slope rock riverbed steel cofferdam. Suspended hoisting foot treatment structures are arranged on suspended hoisting feet of steel cofferdam bottom joints. Each suspended hoisting foot treatment structure comprises two suspended wallboard treatment structures vertically distributed. A suspended wallboard joint section treatment structure in each suspended wallboard treatment structure is an inserting plate type treatment structure or a special-shaped block and inserting plate combined treatment structure. The construction method comprises the steps that firstly, the steel cofferdam bottom joints are spliced and put underwater; secondly, the steel cofferdam bottom joints are conveyed in a floating manner; thirdly, the steel cofferdam bottom joints are sunken; fourthly, previous cofferdam joint sections are mounted and sunken; fifthly, the fourth step is repeated once or multiple times, and sinking of the steel cofferdam is completed; sixthly, steel inserting plates are inserted; and seventhly, bottom sealing structure construction is conducted. By the adoption of the construction method, the suspended hoisting feet of the steel cofferdam can be effectively treated by additionally arranging the suspended hoisting foot treatment structures so as to adapt to the topographical change of a riverbed, the steel cofferdam bottom joints with special-shaped blocks at the bottoms are transferred through a floating conveying device, and the problems about conveying, sinking and bottom sealing construction of the steel cofferdam can be solved effectively.