The delay of analoginput DC merging unit is widely used in HVDC system, and it has an important influence on the DC metering accuracy, control stability and protection rapidity. In order to ensure that the overall delay of analoginput DC merging unit meets the delay requirements, the hardware delay and software delay should be configured reasonably, and the overall delay of the system should be tested. This paper analyzes the delay characteristics of a typical analog input DC combining unit, and analyzes the delay of each link respectively. It is found that the delay of signal conditioning and filtering, A/D conversion and other hardware links are relatively stable, and plays a large proportion of total delay. The specific delay of CPU software link is related to the algorithm actually used and the software execution path caused by the measured parameter state Certain randomness. The experimental results are in good agreement with the theoretical analysis. This paper provides the reference and guidance for the design, manufacture and delay correction of analoginput DC merging unit.