A tri-axis gyroscope with the improved square symmetrical structure consisting of four large proof-masses and four small proof-masses was proposed to realize a monolithic six Degree of Freedom inertial measurement unit. Firstly, the structure and its working principle of the unit were introduced and the required performance was given by detailed design parameters. Then, the modal analysis for in-plane driving mode and out-plane sensing mode was simulated. To increase the mechanical sensitivity for each sense mode, a dual electro-statically tuning method was adopted. In this method, a tuning voltage was first applied on the proof-mass to make the frequency shift between drive mode and yaw mode as small as possible, and the other tuning voltage was applied on a polysilicon electrode plate over the small proof masses to further make the frequency shift among the four modes to be under 30 Hz. By these, the tuning method was verified to be effective. Furthermore, the quality factors for driving and sensing resonant modes were simulated through analysis of the thermoelastic damping mechanism. The analysis results indicate that the quality factors in driving and sensing modes are 23816 and 19507, respectively. Finally, a process flow for high aspect was designed combining the surface and bulk micromachining. The design and simulation for this square symmetrical and decoupled structure demonstrate that the mode matching and quality factor can meet the design requirements for the tri-axis gyroscope.