When GPS signals are lostthe accuracy of the integrated navigation algorithm will decrease.To solve the probleman efficient adaptive smooth switching fusion algorithm is proposed.When the GPS signal is under normal conditionsa high-order integrated navigation algorithm based on GPS measurement is used to estimate UAV attitude.When the GPS signal is lostthe high-order filtering algorithm is terminated and switched to a low-order attitude estimation algorithm based on quaternion.When the GPS signal is restoredthe estimated value of the low-order attitude estimation algorithm is taken as the initial value of the high-order filtering algorithmso as to realize smooth switching between the two attitude estimation algorithms.Simulation results and flight tests show that the switching strategy has good estimation accuracy and feasibilityattitude angle jumping is less than 0.5° during the switching process and the convergence time is less than 0.1 s which effectively improve the accuracy of UAV attitude estimation with or without GPS signals.