Optical sampling coupling technique is one of key techniques in all-optical multiple-input multiple-output (MIMO) signal processing. Both the mode dependent loss (MDL) and the insertion loss (IL) are key parameters which effect the device performance. The condition number of matrix can characterize the performance of optical sampling coupler. The optical sampling coupler which supports all-optical MIMO signal with three linear polarization modes (LP01, LP11a, LP11b) is numerically investigated. Under the condition of three sampling points, the condition number of matrix is used as the optimization condition to optimize the size and the position of sampling spots. MDL can be reduced to 0.03 dB and IL can be reduced to 1.90 dB. In order to further reduce the IL, the number of sampling points is increased. MDL is 0.04 dB and IL is 1.56 dB after the optimization for the size and the position of sampling spots. Although IL can be reduced when we increase the number of sampling points, an optimal number of sampling points exists under the geometric constraints of few mode fibers and sampling points.