The invention relates to a method and apparatus for recycling and reusing waste weak solution and ammonia containing gas of a combined-soda process device. A waste weak solution of a combined-soda process device is boosted by a waste weak solution pump and then delivered to an ultrafiltration membrane assembly and a reverse osmosis membrane assembly so as to form reverse osmosis concentrate and purified permeate; the concentrates formed by the ultrafiltration membrane assembly and the reverse osmosis membrane are mixed, returned to the combined-soda process device, and reused; non-organized discharged ammonia containing gas of the combined-soda process device is delivered to the gas inlet of an ammonia containing gas washing tower by a blower fan; the ammonia containing gas contacts with washing liquid in the tower from opposite directions; when the NH3 content is less than 1 mg/m3, the gas is directly discharged from the top of the tower; when the NH3 content of the solution in the bottom of the washing tower is controlled in a range of 0.1-5%, the solution is divided into two parts; one part is cooled and introduced into the washing tower from the circulation liquid feed inlet, and the other part is delivered to an ammonia preparation reactor to carry out reactions with liquid ammonia and finally is delivered to an ammonia product tank. The provided method and apparatus havethe advantages that the using amount of brine is reduced, at the same time, the ammonia containing gas washing water is used as a raw material to prepare ammonia, and thus the ammonia preparation costis reduced.