In magnetic CVs (polars), the magnetic fields of the white dwarfs are strong enough to prevent materials from the main-sequence companions for forming an accretion disc. Therefore, polars especially eclipsing polars provide a good chance to study mass accretion directly. In the past 4 years, we have monitored several eclipsing polars (e.g., DP Leo and HU Aqr) by using the 2.4-m and 1.0-m telescopes in China and the 2.15-m telescope in Argentina. Nearly 100 eclipse profiles were obtained. In this talk, apart from the detection of a few giant planets orbiting polars, I will summarize some other progresses of our research group at Yunnan Observatories. Our results are as following: (1) the correlation between the out-of-eclipse brightness variation and the change of the eclipse profile suggests that both the accretion hot spot and the accretion stream brighten and become faint instantaneously. This is the direct evidence of variable mass transfer in a CV that is also supported by the relation between the out-of-eclipse brightness and the depth of eclipse. (2) We find the brightness state change is correlated with the dark-spot activity near the L1 point. The low state usually corresponds to the presence of a large spot at L1point, while the dark spot disappear at a high state indicating that it is the dark-spot activity caused the mass transfer in CVs. (3) Magnetic activity cycles of the cool secondary did not correlate with the brightness state change revealing the variable mass accretion was not caused by magnetic activity cycles.