The present disclosure provides a premixer for at least two gases, comprising: a tubular body having a closed end and an opposite, open end; a first flow passage for receiving a first gas, the first flow passage axially extending through the closed end into the tubular body in a sealable manner; a conical tube arranged in the tubular body, wherein a small end of the conical tube communicates with the first flow passage, and a large end of the conical tube extends toward the open end with an edge thereof being fixed to an inner wall of the tubular body, thereby defining a sealed distribution chamber between the tubular body and the conical tube; and a second flow passage arranged on a side portion of the tubular body for receiving a second gas, wherein the second flow passage communicates with the distribution chamber, so that the second gas can be introduced into said conical tube via the distribution chamber in a substantially radial manner. The present disclosure further relates to a radially fixed bed reactor comprising the premixer, a reaction system of oxidative dehydrogenation of butene comprising the racially fixed bed reactor, and a corresponding process.