High-entropy alloys (HEAs) have attracted plenty of attention over the past decade due to their excellent properties. Compared with face-centered cubic (FCC) or body-centered cubic (BCC) structure, hexagonal close-packed (HCP) structure is rarely found in the single-phase random solid solution of HEAs, and most existing HCP HEAs are composed of rare earth (RE) elements. As a novel HEA, Al20Li20Mg10Sc20Ti30 possess two phases including FCC and HCP phase. And the RE elements in Al20Li20Mg10Sc20Ti30 are only one (Sc), showing that it may open up a new way for the design of less-RE HCP HEAs. Moreover, the advantage of low density about 2.67 g/cm3 indicates the tremendous potential of it in aeronautics and prosthetic devices, etc. In this work, the structure stability and thermal properties of face-centered cubic (FCC) and hexagonal close-packed (HCP) Al20Li20Mg10Sc20Ti30 HEAs are studied from density functional theory in which the chemical disorder of completely random solid solution are treated using the special quasi-random structure (SQS). The theoretical lattice constants for both structures are in good agreement with experimental measurements, and HCP phase has better compressibility resistance due to slightly higher bulk modulus. Both HCP and FCC structures are thermodynamic metastable due to slightly positive formation enthalpies and the chemical bond in HCP is more covalent due to its broader pseudogap. Applying Debye-Grüneisen model, thermal properties of HCP and FCC Al20Li20Mg10Sc20Ti30 are studied under different temperatures. Results show that the bulk modulus of both phases decline gently as temperature increases, and bulk modulus of HCP phase is usually larger. By comparison, FCC phase possesses lager volumetric thermal expansion coefficient. Specifically, temperature dependence of thermal entropy for both phases, including vibrational and electronic contribution, is investigated in details, finding that the HCP has a larger entropy than FCC at investigated temperature range, and the entropies of both phases originate predominantly from the vibrational contribution.