The invention discloses a front-opening dual-well-mouth derrick for a sea drilling platform. The derrick comprises a base and a derrick body located between a main well mouth and an auxiliary well mouth. The derrick body comprises a driller opposite side unit, a driller side unit, a main drilling rig side unit and an auxiliary drilling rig side unit, wherein the driller opposite side unit is composed of a transverse pull rod and an inclined pull rod, a racking platform is arranged on the outer side of the driller opposite side unit, a tube drawbench is arranged on the top of the racking platform, the lower section of a track of the tube drawbench is located below a main deck of the drilling platform, and the upper section of the track of the tube drawbench is connected with a cantilever frame stretching out from the derrick body; the side face, opposite to the racking platform, of the driller opposite side unit is open. The derrick is of a four-supporting-leg front-opening structure, the front side face of the driller side unit is open, drilling tools can be conveniently conveyed; the capacity of the racking platform is not affected by the derrick body, the storage capacity of drilling tools is increased, meanwhile, the internal space of the derrick body is saved, the sectional dimension of the derrick body becomes smaller, the size of the derrick body is decreased, and the weight of the derrick body is decreased.