Based on the Electromagnetically-Induced-Transparency (EIT) effect of cesium Rydberg atoms, the dispersion of the probe light will experience a drastically change while the absorption is diminished, as the frequency of it is resonated with that of the corresponding atomic transition. In this case, as the light pulse propagates in the atomic medium, the group velocity of the pulse will be slowed. In the cesium atoms 3-ladder-level system ( $ 6{\rm S}_{1/2}\rightarrow6{\rm P}_{3/2}\rightarrow49{\rm D}_{5/2} $ ),the frequency of the probe light is locked at the resonance transition of $ 6{\rm S}_{1/2}\rightarrow6{\rm P}_{3/2} $ , while the transmission signal of 852 nm probe light is measured by scanning the coupling light frequency near the transition of $ 6{\rm P}_{3/2}\rightarrow49{\rm D}_{5/2} $ , We observed the EIT phenomenon and explored the relationship between the power of coupling laser and linewidth of the EIT signal. The experimental results show that the linewidth of the EIT signal is proportional to the power of the coupling laser. Then under the two-photon resonance condition, the deceleration of the probe light pulse caused by the steep change of the dispersion curve is observed. We also systematically investigate the influences of coupling optical power and temperature of vapor cell on the slowing down of light pulse. The experimental results show that the weaker the coupled light was, the longer the delay time; and the higher the temperature of the atomic gas chamber was, the more obvious the deceleration effect would be observed, those of which are consistent with the theoretical calculations. The investigation of the deceleration of optical pulses based on the Rydberg Electromagnetically-Induced-Transparency is important for understanding the coherence mechanism of 3-ladder-level system and some potential applications, such as in Rydberg-atom-based electric field metrology. This research provides a new tool for the measurement of pulsed microwave electric field through the optical pulse deceleration effect.