The temporal behaviour of micro-discharge in the dielectric barrier discharge (DBD) in air and argon at atmospheric pressure is measured by an optical method using the special set-up of two water electrodes. The nonsymmetrical character of the filament is discovered. Considering the different effects of wall charges on the adjacent micro-discharges in DBD, we have proposed a mapping equation of discharge moment.The calculated results using different parameters show that the decay time constant of wall charges is much larger than an order of 100μs. The results deduced from the mapping equation give a good explanation to the experimental phenomenon that the time interval between two adjacent discharges of a micro-discharge varies between long one and short one alternatively. The variation range of the decay time constant of the accumulated charge is estimated.