The invention discloses a polypeptide medicament sustained release microsphere or a microcapsule preparation with uniform grain size, a preparation method thereof and application. The average grain size of the microsphere or the microcapsule preparation is between 50 nanometers and 100 microns, and the grain size distribution coefficient CV value is less than 20 percent. The polypeptide medicament has a definite structure, has functions of therapy or adjuvant therapy of type-2 diabetes, and is preferably one or more of GLP-1, Exenatide, Exendin-4 and derivatives and analogs thereof. The microsphere or the microcapsule preparation uses a microsphere or a microcapsule with uniform grain size as a substrate to prepare the polypeptide medicament into a sustained release preparation through an embedding mode, and by changing the grain size of the microsphere or the microcapsule, the sustained release cycle is adjustable between one week and one month, and the microsphere or the microcapsule preparation can be applied to the therapy or the adjuvant therapy of the type-2 diabetes and body weight control. Besides, the microsphere or the microcapsule preparation has the advantages of simple preparation process and mild preparation course, and can protect the biological activity of the embedded polypeptide medicament.