We experimentally observe the dynamic evolution of atoms in the evaporative cooling,by in-situ imaging the plugged hole of ultracold atoms.Ultracold rubidium atoms confined in a magnetic trap are plugged using a blue-detuned laser beam with a waist of 20 μm at a wavelength of 767nm.We probe the variation of the atomic temperature and width versus the radio frequency in the evaporative cooling.Both the behaviors are in good agreement with the calculation of the trapping potential dressed by the rf signal above the threshold temperature,while deviating from the calculation near the phase transition.To accurately obtain the atomic width,we use the plugged hole as the reference to optimize the optical imaging system by precisely minimizing the artificial structures due to the defocus effect.