The invention discloses a strapdown 
inertial navigation system(SINS) / global navigation 
satellite system (GNSS) combined navigation 
filter system comprising a primary binding module, an earth parameter calculation module, an initial alignment module, a strapdown inertial navigation calculation module, an improved 
filter algorithm parameter calculation module, a normal-mode combined navigation filtering module, a fault-mode combined navigation filtering module and a feedback correction output module. The improved 
filter algorithm parameter calculation module calculates a sate transition matrix, a 
system drive 
noise covariance matrix, and a measurement 
noise covariance matrix that are needed by an improved kalman filtering algorithem based on data provided by an SINS and a GNSS and transmits the information to the normal-mode combined navigation filtering module and the fault-mode combined navigation filtering module to carry out filtering calculation according to the result and a status flag. According to the invention, the combined navigation filtering 
algorithm is modularized, thereby obviously improving precision of the combined navigation and accelerating the convergence speed of the combined navigation filtering 
algorithm; and the kalman filtering 
divergence can be effectively inhibited.