The implosion dynamics of a magnetically driven capsule-shaped theta pinch plasma is experimentally investigated in this work, focusing on the coupling between the external drive coil and the imploding plasma column. The electron densities were measured by a five-channel optical-fiber interferometer. The local density (or plasma mass) evolution during implosion is obtained by layer-stripping inversion technique. It is observed that plasma mass increases during implosion, which implies that the fixed mass assumption in the classical Lee model is not valid. Here we employ the modified snowplow model with variable mass with momentum conservation constrain to study the dynamic process of implosion. The temporal evolution of magnetic field distribution and plasma current were measured by radial magnetic probe arrays. Plasma current also reveals clear discrepancies from predictions of the conventional Lee model with fixed plasma-to-coil magnetic coupling assumption. An improved variable-coupling model is developed by incorporating time-dependent mutual inductance to explain the experimental phenomena.