The invention discloses an automatic lubricating device. The automatic lubricating device comprises a lubricating mechanism and an oil replenishing mechanism. During lubricating operation, operating personnel unlocks an oil driving piston, and lubricating operation can be automatically finished under the action of a driving spring; after the completion of lubrication, an oil replenishing cylinder is communicated with a lubricating cylinder, the oil replenishing mechanism can be used for automatically finished oil replenishment, and the oil driving piston, a connecting rod and the driving spring are driven to restore to initial positions. During lubrication and oil replenishment, the automatic lubricating device is very easy to operate; moreover, during each oil replenishment and lubrication, the motion travel of the oil driving piston is the same, so that the volume amount of lubricating oil output each time can be kept the same, and a uniform lubricating effect is ensured. Moreover, the lubricating cylinder in the lubricating device is conical, and only one oil flow port positioned in a cone bottom is formed, so that the oil driving piston can be protected from being scratched by the oil flow port in a reciprocating motion process.