The invention relates to an auxiliary device for a bicycle. The device is characterized by comprising a front supporting plate, a rear supporting plate, a right rear rain-shading surface, a right rear support, a zipper, a left rear support, a scroll device, a steering connecting device, a right front rain-shading surface, a front support, a rear support, a front scroll, a rear scroll, a rear scroll clamping groove, a rear hook, a top rain-shading surface, a front hook, a front scroll clamping groove, a front folding device, a rear folding device, a left front rain-shading surface, a left rear rain-shading surface, a right front support and a left front support. The device can be detached and folded, is convenient to install and use, plays a role in shading wind and rain during riding in rainy days, and is economic and practical.