A new square-lattice varying microstructured optical fiber is presented. Using finite-difference time-domain method, the dispersion curves of five-ring varying microstructured optical fiber are caculated, hole-to-hole space of which is 2.0 μm and 2.5 μm, the least diameter is 0.4 μm, 0.6 μm, 0.7 μm, 0.8 μm and 1.0 μm, and the diameter increases from the first ring to the fifth ring by a pitch of Δd=0.1 μm and Δd=0.2 μm respectively. The result shows that the new square-lattice varying microstructured fiber is better than the square-lattice no-varying microstructured fiber in controlling the dispersion and dispersion slope in the wide wavelength, the dispersion curve of which can remain flatter and lower in 1400～2000 nm. When the third ring diameter of varying microstructured fiber has the same value with the diameter of no-varying microstructured fiber, the lowest and flattest dispersion curve is obtained, in this case, the dispersion value is about 30 ps/(km?nm) and Δd=0.2 μm. So the flatter and lower dispersion curve can be realized by adjusting the diameters, distribution of air holes, and the hole-to-hole space.