Due to the high demand of oil exploration and extraction, jack-up rigs, as mobile offshore drilling units are used widely around the world. Its mobile feature makes it practical to be moved in and away drilling sites. To maximize production, jack-up rigs are usually placed adjacent to well head platforms, which already have production wells. This placement can affect the performance of piles of the existing platform, mainly due to degradation of the soil strength around the pile. A study on the jack-up rig leg penetration effect on the piles of the existing platform the jack up rig's approaching is done. The jack-up rig used in this study has 3 legs supporting its structure, which is individually supported by a spud-can on the end of each leg which penetrates the soil under the seabed. The object of the study is to calculate the effect of jack-up rig leg penetration to the piles of the existing platform. The study is done with two jack-up rig approaches, from the north access and from the west access, which will affect 3 out of 4 piles of the existing platform. Only 2 out of 3 jack-up rig legs are considered to affect the platform's pile, which are the legs closest to the platform. Due to its proximity, the penetrating leg of the jack-up rig will have a significant effect in reducing the shear strength of the soil around the jack-up rig leg and spud-can. Soil investigation for soil parameter interpretation is carried after the platform installation. Soil parameter interpretation is carried out from 4 boreholes, representing the locations of each jack-up rig leg penetration site, and the same parameters are used for piles in conjunction with a certain borehole. The jack-up rig penetration depth is first calculated to know the depth of the disturbed zone. Finite element calculation is used to determine the disturbed zone due to jack-up rig leg penetration. From the obtained disturbed zone, the degree of soil strength degradation for each borehole is then calculated to then get the results of the degraded P-Y and T-Z data, used for structural analysis input.