A simulated model has been established to calculate the temperature of helical slow-wave circuits accurately. The finite-element-method software ANSYS was used to analyze the thermal distortion of the traveling-wave-tube (TWT) helical slow-wave circuit. The thermal-analysis results show that the thermal stress deforms the helix primarily at the positions contacting the support rods and that the expansion of the helical tape width and thickness are very small and can be ignored. The effect of thermal strain on the helical slow-wave circuit cold-test characteristics was analyzed in detail. The periodic boundary conditions in the computer code MAFIA were employed to determine the effect on dispersion and on-axis interaction impedance. With increased helix temperature, the phase velocity decreases significantly, and the interaction impedance varies slightly.