Phase structures and transitions of a series of side-chain liquid crystalline polyacetylene (SCLCPA) with a short spacer of three methylene units and different lengths of alkyl tails, namely, poly(5-{[(4'-alkyl-4-biphenylyl)carbonyl]-oxy}-1-pentyne) (P-3,m, m is the number of the carbon atoms in the alkyl tail, m = 5, 7, 9, 11), were investigated using differential scanning calorimeter, polarized light microscopy, and one- and two-dimensional wide-angle X-ray diffraction. With the short spacer, P-3,m possesses the mesogenic groups on the side chain and polyacetylene backbone coupled together and thus renders sheetlike shape with the width nearly twice of the extended side-chain length. Experimental results reveal that the liquid crystalline (LC) structure of P-3,m is strongly dependent on the side-chain tail length, different from that of other SCLCPAs. For m < 11, several layers of the sheetlike P-3,m molecules stack together to form a smectic A (SmA) block, and the number of molecular layers increases with increasing m. The adjacent SmA blocks slide halfway to each other, leading to a highly ordered smectic phase with frustrated molecular packing at low temperatures. The enantiotropic phase transition sequence of P-3,m (m < 11) follows: highly ordered smectic with additional ordering on the subnanometer scale <-> highly ordered smectic <-> semctic C (SmC) <-> isotropic. However, when m is increased to 11, the packing of sheetlike P-3,11 gives the SmC phase, with the transition sequence of SmC with additional ordering on the subnanometer scale <-> SmC <-> isotropic. The phase structures of P-3,m were studied by computer modeling. The phase changing from the highly ordered one to SmC with increasing m may be ascribed to that the P-3,11 molecules intend to maximize the interaction between the biphenyl moieties in neighboring chains. We also investigated the orientation behavior of the highly ordered smectic phase under electric field, wherein a unique striplike texture was well developed, with the side chain and main chain parallel and perpendicular to the electric field, respectively.