The helical once-through steam generator typically consists of multiple helical tubes with varying coiled diameter arranged in parallel. Within these tubes, the process involves two-phase flow of liquid and vapor, and flow instability is a critical thermohydraulic phenomenon that must be addressed. Currently, there is limited research focusing on flow instability in parallel helical tubes with different coiled diameter. Experiments were conducted on parallel helical tubes with different coiled diameter, and the flow instability characteristics of dual-tube, triple-tube and quadruple-tube parallel configurations were investigated respectively. Results indicate that in the triple-tube configuration, flow instability can result in a phase difference of 180°. For the quadruple-tube configuration, variations in flow oscillation amplitudes and phase differences were observed among the tubes. Additionally, the critical heat load for the onset of flow instability increases with the number of tubes, indicating enhanced system stability for configurations with more tubes. The stability hierarchy follows the relationship: dual-tube < triple-tube ≈ quadruple-tube. This study provides novel insights into the stability characteristics of multi-parallel helical tubes and highlights the influence of tube configuration on flow instability dynamics.