The invention relates to a fiber opening device, in particular to a rotary drum-type fiber opening device. The rotary drum-type fiber opening device comprises a rack body, two automatic fiber sucking devices, a plurality of fiber conveying pipes, an opening machine, a fiber bin and an automatic oil spraying mechanism, wherein the automatic oil spraying mechanism comprises a feeding hopper, a sliding sleeve, a shell, an overturning inner drum, a stirring shaft and a planetary gear set, the feeding hopper is communicated with the fiber conveying pipes, one end of the sliding sleeve is communicated with the feeding hopper and is in rotary connection with the feeding hopper, the other end of the sliding sleeve is communicated with the overturning inner drum and is in rotary connection with the overturning inner drum, the shell is fixed to the rack body, oil spraying heads are arranged on the inner wall of the shell, the overturning inner drum is arranged in the shell, an oil inlet hole is formed in the overturning inner drum, the overturning inner drum is fixedly connected with a planetary carrier of the planetary gear set, the overturning inner drum is communicated with the fiber conveying pipes, and the stirring shaft is arranged in the overturning inner drum. For solving the problem of uneven fiber and oil spraying, the device can uniformly spray fiber and oil.