Concentration of O_2 and CO_2 has a great effect on storage quality of cold stored fruits or vegetables. Obtaining concentration variation and distribution characteristics of O_2 and CO_2 concentration in a controlled atmosphere storage (CA storage) can provide theoretical basis for optimal design and operation of CA storages. Hence, a combination of CFD calculation and experimental study were carried out to study the complicated phenomenon of air flow, heat and mass transfer in CA storage. An unsteady, 3D (three-dimensional) global CFD model was established for a real CA storage for apples in Xian. The CFD model includes k-epsilon turbulent model and species transport model. The apple zone was regarded as porous medium zone. Air cooler was equipped in the storage. User-Defined-Function (UDF) programs were developed in C language to achieve the variation of apples' respiration rate and mass fractions of gas species at the velocity-inlet boundary along with the local O_2 and CO_2 concentration in the CA storage. The mass transfer phenomenon for the reduction process of O_2 concentration was numerically studied. The calculated concentration variation of O_2 and CO_2 were validated by experimental results. The experimental system consisted of a gas probe, gas analyzer and accessory measuring device. The gas probe was set inside the gas zone of the CA storage and connected to a gas analyzer at test room, where the piece of gas from the probe was measured and analyzed in every two hours. The numerical results were compared with the experimental data for whole reduction process of O_2 concentration, including the reduction process of O_2 concentration when N_2 generator was working followed by the reduction process due to apples' aerobic respiration when N_2 generator was stopped. The numerical results from the displacement process of O_2 by inputting N_2 indicated that the concentration of O_2 in gas zone decreased in an exponential function, which was validated by experimental results with a deviation of 0.027. The numerical results from the following reduction process due to apples' aerobic respiration were validated with the average differences of 3.06% for O_2 and 16.2% for CO_2 concentration. Moreover, increasing N_2 concentration of N_2 generator would shorten the reduction process of O_2. In the case of studied CA storage, O_2 concentration decreased faster in gas zone than in apple zone in the beginning 4.1 h, but appeared the opposite characteristic afterwards. Concentration of CO_2 increased similarly in gas zone and in apple zone. Concentration of CO_2 decreased gradually with time. The concentrations of O_2 and CO_2 in gas zone appeared to be relatively uniform, but there existed concentration gradients in apple zone. The proposed 3D unsteady model can be used to get the reasonable reduction time of O_2 concentration for CA storage, which would provide a valuable reference for choosing reasonable N2 generator.