To study the guardring suppression effect on the extension of the photosensitive area in planartype front illuminated InP/InGaAs heterostructure detector，the InGaAs photo detectors with different distances between guardring and PN junction were designed and fabricated. The actual distance between guardring and PN junction of the detector was calculated based on the atomic force microscopy (AFM) and scanning capacitance microscopy (SCM) measurements. The characteristics of the photo response of the detectors with guardring were carried out with laser beam induced current (LBIC) method. It was indicated that LBIC signal of the photo detector without guardring fit well with the exponential decay function, while that of detector with guardring followed the Gaussian distribution. The extension value of the photosensitive area decreased linearly as the distance between the guardring and PN junction decreased. It was concluded that the appropriate gap between the guardring and PN junction should be in the range of 7 to 12 μm.