A drive force transmission device of double-needle sewing machine can transmit the drive force of the main shaft individually to two parallel needle stems which are liftably supported in a needle stem bearing frame respectively. The device is characterized in that: it comprises: clamping recessed parts formed on the upper and lower side of every needle stem with predefined intervals; longitudinal grooves formed on the recessed parts on the upper and lower side of every needle stem; motion rods each is rotatably supported in the longitudinal groove by a pin on the middle section along the length direction and has two projected abutting surfaces respectively on the upper and lower end to alternatively block the recessed part on the upper or lower side; a needle stem crank connected with the main shaft; a needle stem holder mechanism which is connected with the needle stem crank, and has a first clamping piece capable of clamping with the clamping recessed part on the lower side of every needle stem, a first force application member for forcing the first clamping piece towards the clamping side and a locking mechanism for locking the first clamping piece on an un-clamped position; a needle stem clamping mechanism which is disposed on the needle stem bearing frame, and has a second clamping piece capable of clamping with the clamping recessed part on the upper side of every needle stem and a second force application member for forcing the second clamping piece towards the clamping side; an operating mechanism used for selectively releasing the first clamping piece of the clamping recessed part on the lower side of a needle stem and unlocking the locking mechanism when the needle stem is positioned above the motion stroke; and a drop prevention apparatus used for limiting further movement and preventing the needle stem from dropping off from the needle stem bearing frame.