The non-ideal pipe flow would induce unsteady fluctuation under the condition of turbulence, in which large scale fluctuation would influence the measurement accuracy of ultrasonic flowmeter. A pack-of turbulent dissipation grilles was designed paralleling to the acoustical path of the ultrasonic flowmeter to limit the scale and intensity of turbulent fluctuation. The turbulence reduction grilles can optimize the flow field from two aspects: reducing the vortex intensity and controlling the turbulence pulsation scale. And the diameter of the grid bar was close to 1mm, which had little effect on the average flow velocity. A numerical simulation based on detached eddy simulation (DES) was used to study the unsteady internal flow and the simulation was verified by laser Doppler velocimetry (LDV) experiment. The design was compared with the original setup, and the design did not disturb averaged flow field due to the small diameter of the turbulent dissipation grilles. For vorticity magnitude, the turbulent dissipation grilles could smooth the flow and restrict the vorticity magnitude around the acoustical path. In the range of L＝10~50mm, the vorticity magnitude of the design was decreased significantly. For fluctuating scale, the turbulent dissipation grilles broke the mechanism which turbulence propagated toward larger scale. When Re was greater than 10000, the turbulence reduction optimization had a significant effect on the reduction of turbulence error. When Re was greater than 40000, the turbulence error was about half of that before optimization. The introduction of turbulent dissipation grilles had a promising prospect for the improvement of the time domain resolution of ultrasonic measurement and the design of low power consumption.