We describe a new method for side pumping of double-clad fiber lasers through trapezoidal micro-prism, which can pump the light sources from laser diodes, especially laser diode arrays, into double-clad fibers easily and effectively. We use a unique-designed trapezoidal micro-prism with the same refractive index as the inner cladding of double-clad fiber. The pump light can be coupled into the inner cladding of double-clad fiber via reflection of the coated side of the trapezoidal micro-prism. Advantages of this technique include no destruction of inner-cladding, high pumping efficiency, simplicity, low cost, easy industrialization and so on. The mechanism and the application method is first introduced and the theoretical coupling efficiency was given (up to 90 %). We analyze the parameters of the trapezoidal micro-prism and the light sources including line width, angle of divergence, etc.. Then we make a further discussion of its applied areas including laser diode, pigtailed laser diode, laser diode arrays, different shape of inner cladding, etc.