The invention relates to the technical field of chemical industry production, and in particular relates to a classified recycling system and method of dilute formaldehyde in paraformaldehyde production. The system is characterized in that a formaldehyde solution storage tank is connected with an evaporator through a pipeline; the evaporator is connected with a distillation still through the pipeline; the distillation still is connected with a dryer through the pipeline; the dryer is connected with an absorption tower through the pipeline; the absorption tower is connected with a precipitation tank through the pipeline; a turbid solution outlet of the precipitation tank is respectively connected with the evaporator and a urotropin workshop through the pipeline; and a clear solution outlet of the precipitation tank is connected with a formaldehyde production workshop through the pipeline. The mixed solution containing dilute formaldehyde and dust in the absorption tower are precipitated, filtered and separated through the precipitation tank, and the upper clear solution is returned to the formaldehyde workshop to be recycled as soft water; and the lower turbid solution is returned to the urotropin workshop to produce urotropin, or to continuously produce paraformaldehyde at a paraformaldehyde poly-concentration section. Through the adoption of the system and method provided by the invention, the yield of formaldehyde is increased, and the economic loss of the urotropin workshop is reduced.