The invention discloses a definition method for an initial gesture of a marine optical fiber gyro strapdown system, comprises first, preparing preheat for a SINS, second, adjusting the SINS to enter into a rough alignment period for an initial alignment after the preheating preparation of the SINS is finished, collecting a marine gyroscope output and an accelerometer output, calculating three error misalignment angle phi xn, phi yn, and phi zn between a geographic coordinate system and a real geographic coordinate system through a rough initial strapdown gesture matrix which is obtained after the rough alignment is finished, entering into an extractive alignment when the three error misalignment angle are less than 10 degree, if not, the rough alignment is need to be renewed, third, the extractive alignment, continuously collecting the marine gyroscope output and the accelerometer output on the basis of the rough alignment, adopting an optimal control technique of a linear quadratic form LQ, and obtaining optimal correcting controlling angle speed which is needed in the process of the extractive alignment. The invention seeks a tradeoff between time and aligning accuracy, the aligning accuracy of an initial gesture resolving proposal is equal to a traditional proposal, but aligning time and convergence speed are greatly increased.