This article comes from 2022 IYPT’s problem. Main researches are properties of flow field around a candle and the paper spiral’s torque. This article consults literature and uses data to verify the properties of flow field around a candle. The pictures of flow field are simulated by COMSOL Multiphysics and flow visualization is realized striation technique. Then two different methods are used to derive influencing factors of paper spiral’s torque. An expression for paper spiral’s rotational inertia comes from integration method. Experimental data are used to fit the curve of paper spiral’s rotation. Angular acceleration can be obtained from the curve. Finally calculate paper spiral’s torque and analyze relationship between paper density, suspension height, layers of spiral and the torque. Data also prove the theory and calculation method are rational and feasible.