Ge can be converted into direct bandgap material under tensile strain. Pyramid-shaped Ge quantum dots (QDs) on GaSb system is proposed. The strain distribution and band structures are investigated with different QD sizes. Flat Ge QDs are desirable for indirect to direct band gap conversion. Light emission from 1.8 μm to 5.8 μm can be achieved from Ge/GaSb QDs with the width ranges from 4 to 14 nm and the height ranges from 1 to 5 nm. A 3.1 μm laser with Ge/GaSb QDs with height of 3 nm and width of 12 nm is designed and the device performance is simulated at room temperature. The QD size fluctuations are also considered. Under the root mean square of QD size fluctuation of 0.22, the calculated optimal optical confinement factor, thickness of the waveguide, surface density of QDs are 0.013, 0.366 μm and 3.8×1010 ^cm-2^, respectively, while the minimum threshold current density is 28.98 A/cm2^. This work provides a feasible way for the fabrication of mid-infrared lasers.