Abstract: ‍ ‍Based on the consideration that the information freshness has a great impact on the design and deployment of the wireless sensor status update system， the paper considers a status update system made up of two sensors and a common destination node. The two sensors， equipped with one unit buffer respectively， are capable to sense the update of corresponding physical process according to the Bernoulli process in every slot. Subsequently the sensors will deliver the updates in the form of short blocklength packet. We adopt a recently proposed metric， the age of information （AoI）， to portray the freshness of the updates. Initially， being centered on the combination of each sensor buffer state， the system can be modeled as a continuous-time finite-state Markov process， where finite-state represents the different states of the buffers and continuous-time denotes the evolution of the age-related processes. Secondly， the average AoI could be estimated by the stochastic hybrid system （SHS）. Finally， the simulation results suggest that， when the successful sensing probability of packet is 0.2， the average AoI decreases by 20.6% when there is an increase of SNR from 0 dB to 3 dB.And when the SNR is fixed on 4 dB， the average AoI decreases by 8.8% in the event that the blocklength of update increases from 300 channel uses to the 400 channel uses. The research has laid a foundation and provided theoretical basis for the subsequent design of the buffer-aided status update system.