By fused biconical tapering at the fusion point of single mode single-core fiber and single mode twin-core fiber, a power tapered fiber coupling technique is implemented. The difficulties in direct-coupling with light source and single made fiber and signal detection in the two cores due to its special structure are solved. The analysis about the coupling theory of single mode single-core fiber and single mode twin-core fiber is given. With the combination of direct-coupling and weakly coupled-mode theory, the coupling equations of single mode single-core fiber and single mode twin-core fiber are proposed. The factors which influence the coupling power are also discussed. This theory analysis can explain the coupling character effectively.