The invention provides a preparation method of a TEM sample of metal material thin plate and slice cross section. The method is characterized in that: the cross section sample whose thickness is mechanically reduced to 30-50 micrometers is placed in a pit on a conducting plate of a main clamping plate of a general twin-jet electropolishing device clamp, a non-conducting thin plate is placed at the outside of the sample for completely shielding a circular aperture of 2-2.5mm on the main clamping plate, and a micropore with a diameter which is less than the length of the cross section sample on the center of the non-conducting thin plate forms a twin-jet electropolishing area whose diameter is equal to the diameter of the micropore on the cross section sample. If a hole appears on in the area, a photosensitive control unit is immediately induced for action, and the twin-jet electropolishing process automatically stops, so that a thin region with the largest area for electrons to penetrate is obtained. The method for preparing the TEM sample of metal material thin plate and slice cross section on the general twin-jet electropolishing device can be simply and easily carried out; the area of the thin region for electrons to penetrate is substantially increased, and reliability of the obtained large-area thin region for electrons to penetrate is greatly improved.