The invention relates to the field of subgrade side ditch construction, in particular to a construction method for continuously casting a subgrade side ditch through a vibrating and stirring type automatic slip form. The problems that in the subgrade side ditch construction process, traditional processes are low in efficiency, and the quality is difficult to control are solved. The method comprises the steps that site formation is conducted; a setting bucket is adopted for achieving one-step ditch excavating and forming; the vibrating and stirring type automatic slip form is hoisted; the vibrating and stirring type automatic slip form extrudes poured concrete through a hydraulic oil cylinder in the vibrating and stirring type automatic slip form, the vibrating and stirring type automatic slip form travels through reverse thrust generated by the hydraulic oil cylinder and conducts continuous operation motion, and the ditch is cast and formed at a time; manual leveling and smoothing are conducted; and settlement joints are formed. The compactness and pouring quality of concrete are improved through the concrete extrusion force generated through automatic travelling of the slip form and an attached vibrator of the slip form. The ditch is cast and formed in the travelling process, the links of manual repeated mounting, dismounting and form transferring and waiting for concrete setting are not needed, the construction efficiency is greatly improved, and the construction period is greatly shortened.