A bag with adjustable firmness and a using method thereof, which relate to the field of travel goods. The bag includes a bag body and an inflatable airbag. An airbag accommodating space is arranged around an inner side of the bag body. The inflatable airbags are arranged in the airbag accommodating space and around the inner side of the bag body in a circle with space between each other. Inflation of the airbag increases the firmness of bag panel to achieve structured and three-dimensional shape for the bag body. Deflation of the bag decreases the firmness of bag panel to allow bag body to be softer and easier to store. The expansion or contraction of airbag can adjust the firmness of bag body according to actual usage scenarios, solving the problem in conventional bags where flexibility and rigidity of fabric cannot be adjusted to suit different usage needs.