The invention provides a sewage treatment agent prepared by using solvent-based lignin or derivatives thereof. The raw materials are as follows: solvent-based lignin or derivatives thereof, aldehyde, urea, a base catalyst and an acid regulating agent; the preparation comprises the following steps of: mixing the lignin and derivatives and water, adding alkali solution to adjust the pH, adding the aldehyde, heating, reacting, adding the urea with the needed using amount, continuously reacting, also being capable of adding the urea by two times according to the needs, adding acid to adjust the pH, carrying out polycondensation, adding alkali to adjust the pH, adding the post urea, reacting, adding the acid regulating agent to adjust the pH, depositing, extracting and filtering, drying and obtaining the sewage treatment agent product. The invention can utilize the renewable biological resources reasonably, thus not only solving the defects that the existing sewage treatment has high cost and lower efficiency, but also being capable of replacing partial of petrochemical materials; and the sewage treatment agent of urea formaldehyde modified derivatives prepared by using solvent-based lignin is used for treating the sewage containing dye or heavy metal ions and has good using effect.