This study aims to describe the improvement of students' algebraic reasoning in solving generating function problems based on implementing the lesson study-based snowball throwing learning model. The subjects in this study were students of the odd semester STKIP PGRI Sumenep mathematics education study program who had passed courses in mathematical logic and number theory. This research is a type of classroom action research with a qualitative approach. Lesson study and CAR have many similarities in terms of the stages, namely planning (plan), implementation & observation (do), and reflection (see). The instruments in this study were problem-solving task sheets and lesson study observation sheets. Data were analyzed using quantitative descriptive analysis by describing the percentage of quantitative data obtained. On the indicator looking for patterns, there was an increase of 50%, on the indicator finding patterns there was an increase of 43.75%, while on the generalization indicator there was an increase of 45%. This shows that implementing the lesson study-based snowball throwing learning model can improve students' algebraic reasoning in solving generating function problems.