This is a preliminary study of design research that investigates preservice mathematics teachers' proof level and the possible task of scaffolding-based interventions in proving the triangle theorem. The research subjects consisted of 58 second-semester mathematics education students at Universitas Negeri Surabaya, Indonesia. This research is descriptive using quantitative and qualitative approaches. Data collection uses a test to determine the level of proof of prospective mathematics teachers based on Miyazaki's classification. This method classifies four levels in constructing a proof, mainly Proof A, Proof B (deductive), Proof C, and Proof D (inductive). The results showed that there were 38% of students' answers in constructing proof with level Proof A, 5% of students' answers in constructing proof with level Proof B, 15% of students' answers in constructing proof with level Proof C, and the remaining 42% of students' answers in constructing proof with level Proof D. Furthermore, the scaffolding-based intervention task refers to the preservice teacher's difficulties in proving the triangle theorem, including a lack of understanding of concepts, not understanding language and mathematical notation and difficulties in starting proofs.