In this paper, we utilize a classical ensemble model with Heisenberg-core potential to study sequential double ionization (SDI) of Ar atom by an elliptically polarized laser field. The results show that for random laser phases, as the laser wavelength increases, the ion momentum distribution gradually evolves from a six-band structure at 800 nm to an eight-band structure at 1600 nm. When the laser phase is stable, the ion momentum distribution from 1600 nm laser field exhibits a ten-band structure. These multi-band structures directly reflect the subcycle ionization dynamics of electrons in an elliptically polarized laser field. There is a significant shift among the outer three bands of ion momentum distrbutions from different laser phases, which leads to the fact that only one band is observed in the outer region of the ion momentum distribution for the case of random laser phases. By analyzing the ionization times of the two electrons, it is found that for the case of random phases, the inner bands of the ion momentum distributions originate from those combinations of electron ionization bursts with the ionization time difference of 0.5 cycle, and the outer bands arise from those combinations of ionization bursts with the ionization time difference of 1, 2 and 3 cycles. For 800 nm, the middle band corresponds to those combinations of ionization bursts with the ionization time differences of 1.5 and 2.5 cycles. For 1600 nm, there are two bands in middle regime. One is from the combination with the ionization time difference of 1.5 cycles, and the other is from those combinations with the ionization time difference of 2.5 and 3.5 cycles. These results indicate that in the case of long wavelength and phase-stable laser, the subcycle dynamics in sequential double ionization of atoms is more likely to be observed.