For　the　on-orbit　space　zoom　camera,　the　camera　focal　length　s　in　a　constant　process　of　change;　accordingly,　compared　with　calibrating　other　camera　intrinsic　parameters,　calibrating　the　focal　length　has　a　practical　significance　for　the　space　zoom　camera.　With　the　vanishing　points　obtained　from　the　solar　panel　of　human-made　space　satellites,　this　paper　introduces　a　focal　length　self-calibration　method　for　the　on-orbit　space　zoom　camera.　First,　the　geometrical　relationship　and　infinite　homography　of　vanishing　points　at　various　camera　positions　are　used　to　derive　the　method.　To　improve　the　accuracy　and　robustness　performance　of　this　approach,　an　optimization　method　is　then　proposed　to　nonlinearly　optimize　the　camera　focal　length.　Finally,　simulation　and　real　physical　experiments　demonstrate　that　the　proposed　method　is　flexible　and　accurate　with　good　anti-noise　interference　and　real-time　capacity.　The　method　proposed　in　this　paper　makes　more　realistic　sense　for　a　number　of　important　space　tasks.　(C)　2019　Optical　Society　of　America