The solar Hα space telescope is the first space observation telescope to realize the full-surface Hα monochromatic image in the world (including the three bands of Hα core and wire wing), which is of great significance for solar physics research. The signal-to-noise ratio (SNR) is an important index for quantitative evaluation of imaging quality and working stability of space telescope, so it is very important to calculate and estimate SNR for the development and use of solar Hα space telescope. The calculation model of the signal-to-noise ratio is established and the formula is derived according to the principle of the solar Hα space telescope and the way of energy transfer. On this basis, the interval of the signal-to-noise ratio and the exposure time is calculated, and the exposure time of the single pixel in the best working condition is 5 ms, and the exposure time of the pixel combination is 10 ms. It has been experimentally verified that the calculated and measured values are within one percent of the error. This can verify the accuracy and practicability of the signal-to-noise ratio calculation model.