The invention discloses a machine for constructing grouting gravel piles in a tunnel and a constructing method. The machine for constructing the grouting gravel piles in the tunnel comprises a drilling system, a grouting system, a walking control room and a crawler system. The drilling system comprises a winch, a host, a constructing machine frame, a drill stem and a motor system. The winch is connected with the constructing machine frame. The drill stem is connected with the constructing machine frame. A thin-walled steel sleeve is arranged on the outer surface of the drill stem. The groutingsystem comprises a grouting pressure system, a conveying hose and a grout storing system. The operation method of the constructing machine includes the steps of constructing positioning, drilling constructing, connecting constructing, gravel putting-in, grouting constructing and machine moving. The machine for constructing the grouting gravel piles in the tunnel integrates a drilling constructingmachine and a grouting machine, overcomes the difficulties of height limit and space deficiency in the tunnel, solves the problem of once drilling and pile-forming difficulties in the prior art, meanwhile avoids the problem of the high hole collapsing probability in the oblique file-forming process, and further improves the shear capacity of the grouting gravel piles.