In order to reduce the floor area and unit cost，the layout of newly-built coal-fired units in China is mostly very compact in recent 20 years. Among them，changing the front coal bunker to the side coal bunker has become one of the common choices for coal-fired units. The layout of side bunker brings two problems：one is that the pipeline of remote pulverizing system is long，which is easy to lead to uneven distribution of air and powder;Second，the primary air duct is greatly shortened，the air volume measuring device does not have a suitable installation position，and the accuracy of primary air measurement decreases. In order to solve the accidents such as large fluctuation of combustion and air distribution，powder blockage at the outlet of the pulverizer，ignition and excessive NO_x concentration at the outlet of the furnace of 660 MW ultra super-critical side coal bunker coal-fired boiler in a power plant due to the distortion of primary air measurement of the medium speed pulverizer，the flow field of the circular primary air pipe at the inlet of the pulverizer was studied based on the(fluent）numerical simulation software. The primary air flow distribution in the original design and after straightening the air duct were calculated，respectively. The results show that there is an obvious transition point in the primary air velocity distribution at 1 to 3 times the diameter after the pipe turns，with the increase of primary air flow. However,the air volume measurement before and after this point is distorted，resulting in large fluctuation of air volume，when the inlet air velocity is 9.5 m/s. By straightening the inlet pipe，the flow field of the primary air duct is more regular，and the transformation of the flow field tends to be gentle，avoiding the distortion of the primary air measurement. By installing rectifying device，moving the air volume measurement device to the bottom of air powder pipe and appropriately extending the length of straight pipe section，the distortion of air volume measurement can also be avoided to a great extent. If the site space does not meet the conditions，avoiding the components or structures that are easy to cause poor flow stability such as tee，turn，reducer or valve in the short pipeline，and also avoid the impact of distortion of primary air volume measurement on unit disturbance.