The invention relates to a water-soluble plant growth nutrient solution and a preparation method. In the plant growth nutrient solution, the content of small molecular weight water-soluble chitosan accounts for 8-120g/L, the sum of the content of nitrogen, phosphorus and potassium is larger than or equal to 60g/L, the sum of the content of medium trace elements accounts for 3-30g/L, the content of organic matter accounts for 30-200g/L, the content of plant growth regulator accounts for 0.01-5g/L, and the content of spread penetrating agents is larger than or equal to 0.2g/L; the small molecular weight water-soluble chitosan is obtained through the preparation steps that chitosan is firstly subjected to alkalization of strong base, then subjected to carboxyl chemical modification, degraded through strong oxidant after carboxyl chemical modification and finally neutralized through acid. The water-soluble plant growth nutrient solution integrates the effects of nutritional complement, growth promotion, antibiosis, virus resistance, stress tolerance and the like and has multiple functions and effects of improving soil, preventing hardening, increasing ground temperature, resisting to disease, being tolerant to stress, resisting to continuous cropping, supplementing nutrition rapidly, raising the yield and improving fruit quality and the like.