B and V light curves of an EA-type binary V205 in the globular cluster NGC 5139 are analyzed by the W-D program. We found that V205 is possibly a detached binary and the mass ratio is 0.1596. The secondary component is touching or nearly touching its inner Roche Lobe. By studying the O-C diagram of V205, we discovered that the orbital period is continuously decrease at a rate of dp/dt = 1.89(+/- 0.01) x 10(-7) d yr(-1) and should be caused by angular momentum and mass loss. The angular momentum loss will drive it evolve into a contact binary. Since V205 is a proper motion member of NGC 5139, we estimated its absolute parameters based on the distance modulus of the cluster and determined that: a = 2.50R(circle dot), M-1 = 0:76M(circle dot), R-1 = 1,14R(circle dot), L-1= 5.46L(circle dot,) M-2 = 0.12M(circle dot), R-2 = 0.52 R-circle dot,R- and L-2 = 0.70 L-circle dot. V205 occupied the blue straggler stars on the color-magnitude diagram of NGC 5139. It is an eclipsing blue straggler and is most possibly formed by mass transfer between the two components. Since original short-period systems similar to V205 should be evolved in such a long life time of the globular cluster, the short period binary should undergo special evolutionary stages. High accuracy photometric and high resolution spectral observations are essential for this unusual system. (C) 2017 Elsevier B.V. All rights reserved.