The in-plane magnetic penetration depth, λ_m(T), was measured in single crystals of SrPd_2Ge_2 superconductor in a dilution refrigerator down to T=60 mK and in magnetic fields up to H_dc = 1 T by using a tunnel diode resonator. The London penetration depth, λ, saturates exponentially approaching T→ 0 indicating fully gapped superconductivity. The thermodynamic Rutgers formula was used to estimate λ(0) = 426 nm which was used to calculate the superfluid density, ρ_s(T)=λ^2(0)/λ^2(T). Analysis of

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... ρ_s(T) in the full temperature range shows that it is best described by a single - gap behavior, perhaps with somewhat stronger coupling. In a magnetic field, the measured penetration depth is given by the Campbell penetration depth which was used to calculate the theoretical critical current density j_c. For H < 0.45 T, the strongest pinning is achieved not at the lowest, but at some intermediate temperature, probably due to matching effect between temperature - dependent coherence length and relevant pinning lengthscale. Finally, we find a compelling evidence for surface superconductivity. Combining all measurements, the entire H-T phase diagram of SrPd_2Ge_2 is constructed with an estimated H_c2(0)=0.4817 T.