Coherent beam combining of fiber lasers can achieve high power laser with good beam quality. Most studies on coherent beam combining focus on phase controlling,i.e.,how to make the beamlets coherent. Beam combining of the coherent beamlets,i.e.,how to improve the fill factor of the coherent laser array,is also an important factor that affects the beam quality,yet it has seldom been taken into account. In order to ensure the beam quality of the coherently combined beam,the aperture filling technique should be employed to improve the fill factor of the fiber laser array. Recent progress on the aperture filling technique,i.e.,amplitude-to-phase technique,self imaging waveguide,beam shaping,diffraction optics and beam truncation was introduced. Each technique was analyzed and evaluated in detail. It is shown that the beam truncation technique is free of complicated optical system design and easy to implement,which holds great potential in coherent beam combining of high power fiber laser array.