The invention discloses an investment-saving ammonia synthesis process and device. The device comprises an internal heat exchange type ammonia synthesis tower, and an inlet/outlet heat exchanger, wherein synthetic tower inlet gas and synthetic tower outlet gas which enter and exit the internal heat exchange type ammonia synthesis tower are subjected to heat exchange through the inlet/outlet heat exchanger; the inlet/outlet heat exchanger is divided into a high-temperature section and a low-temperature section; part of the synthetic tower inlet gas is extracted out from an outlet of the low-temperature section of the inlet/outlet heat exchanger and is fed into a shell pass annular space of the internal heat exchange type ammonia synthesis tower; the residual synthetic tower inlet gas is extracted out from the high-temperature section of the inlet/outlet heat exchanger and is fed into the internal heat exchange type ammonia synthesis tower; a shell and the interior of the internal heat exchange type ammonia synthesis tower have different design temperatures; the design temperature of the shell is lower than that of the interior part of the tower. The invention discloses the ammonia synthesis process and device which can save more investment compared with the prior art.