In order to elucidate the flow and pressure drop characteristics associated with condensation heat transfer on the shell side of a helical tube heat exchanger, a three-dimensional transient numerical simulation model was developed. Utilizing this model, a detailed analysis of the shell-side fluid flow characteristics was performed. Numerical simulations were conducted under various combinations of mass flux (G) and heat flux (q_wall) to investigate their influence on shell-side condensation heat transfer and pressure drop behavior. The results show as follows. For constant heat flux: as the mass flow rate increases, the surface heat transfer coefficient h_s gradually decreases, with a progressively slower rate of decline. The Nusselt number Nu increases with mass flow rate. For low heat flux (≤ - 50 kW/m²), the growth rate of Nu initially decreases and then increases, whereas for high heat flux (> - 50 kW/m²), the growth rate initially rises before declining. For constant mass flow rate:increasing the heat flux results in an increase in h_s, while Nu also rises, with a greater growth rate as mass flow rate increases. The pressure gradient (dP/dl) initially increases and then decreases with rising vapor quality, with a critical turning point occurring at a vapor quality of approximately 0.8. This observation highlights the strong interplay between the pressure gradient, vapor quality, and flow regime.